Miscellaneous documentation fixes ...

This patch gathers miscellaneous minor fixes to the documentation, and comments
in the source code.

Change-Id: I631e3dda5abafa2d90f464edaee069a1e58b751b
Co-Authored-By: Soby Mathew <soby.mathew@arm.com>
Co-Authored-By: Dan Handley <dan.handley@arm.com>

Optimize EL3 register state stored in cpu_context structure ...

This patch further optimizes the EL3 register state stored in
cpu_context. The 2 registers which are removed from cpu_context are:

  * cntfrq_el0 is the system timer register which is writable
    only in EL3 and it can be programmed during cold/warm boot. Hence
    it need not be saved to cpu_context.

  * cptr_el3 controls access to Trace, Floating-point, and Advanced
    SIMD functionality and it is programmed every time during cold
    and warm boot. The current BL3-1 implementation does not need to
    modify the access controls during normal execution and hence
    they are expected to remain static.

Fixes ARM-software/tf-issues#197

Change-Id: I599ceee3b73a7dcfd37069fd41b60e3d397a7b18

Remove coherent stack usage from the warm boot path ...

This patch uses stacks allocated in normal memory to enable the MMU early in the
warm boot path thus removing the dependency on stacks allocated in coherent
memory. Necessary cache and stack maintenance is performed when a cpu is being
powered down and up. This avoids any coherency issues that can arise from
reading speculatively fetched stale stack memory from another CPUs cache. These
changes affect the warm boot path in both BL3-1 and BL3-2.

The EL3 system registers responsible for preserving the MMU state are not saved
and restored any longer. Static values are used to program these system
registers when a cpu is powered on or resumed from suspend.

Change-Id: I8357e2eb5eb6c5f448492c5094b82b8927603784

Allow FP register context to be optional at build time ...

CTX_INCLUDE_FPREGS make variable allows us to include or exclude FP
registers from context structure, in case FP is not used by TSPD.

Fixes ARM-software/tf-issues#194

Change-Id: Iee41af382d691340c7ae21830ad1bbf95dad1f4b

Initialise CPU contexts from entry_point_info ...

Consolidate all BL3-1 CPU context initialization for cold boot, PSCI
and SPDs into two functions:
*  The first uses entry_point_info to initialize the relevant
   cpu_context for first entry into a lower exception level on a CPU
*  The second populates the EL1 and EL2 system registers as needed
   from the cpu_context to ensure correct entry into the lower EL

This patch alters the way that BL3-1 determines which exception level
is used when first entering EL1 or EL2 during cold boot - this is now
fully determined by the SPSR value in the entry_point_info for BL3-3,
as set up by the platform code in BL2 (or otherwise provided to BL3-1).

In the situation that EL1 (or svc mode) is selected for a processor
that supports EL2, the context management code will now configure all
essential EL2 register state to ensure correct execution of EL1. This
allows the platform code to run non-secure EL1 payloads directly
without requiring a small EL2 stub or OS loader.

Change-Id: If9fbb2417e82d2226e47568203d5a369f39d3b0f

Add build configuration for timer save/restore ...

At present, non-secure timer register contents are saved and restored as
part of world switch by BL3-1. This effectively means that the
non-secure timer stops, and non-secure timer interrupts are prevented
from asserting until BL3-1 switches back, introducing latency for
non-secure services. Often, secure world might depend on alternate
sources for secure interrupts (secure timer or platform timer) instead
of non-secure timers, in which case this save and restore is
unnecessary.

This patch introduces a boolean build-time configuration NS_TIMER_SWITCH
to choose whether or not to save and restore non-secure timer registers
upon world switch. The default choice is made not to save and restore
them.

Fixes ARM-software/tf-issues#148

Change-Id: I1b9d623606acb9797c3e0b02fb5ec7c0a414f37e

Reduce deep nesting of header files ...

Reduce the number of header files included from other header
files as much as possible without splitting the files. Use forward
declarations where possible. This allows removal of some unnecessary
"#ifndef __ASSEMBLY__" statements.

Also, review the .c and .S files for which header files really need
including and reorder the #include statements alphabetically.

Fixes ARM-software/tf-issues#31

Change-Id: Iec92fb976334c77453e010b60bcf56f3be72bd3e

Place assembler functions in separate sections ...

This extends the --gc-sections behaviour to the many assembler
support functions in the firmware images by placing each function
into its own code section. This is achieved by creating a 'func'
macro used to declare each function label.

Fixes ARM-software/tf-issues#80

Change-Id: I301937b630add292d2dec6d2561a7fcfa6fec690

Add helper library for cpu context management ...

This patch introduces functions for saving and restoring shared system
registers between secure and non-secure EL1 exception levels, VFP
registers and essential EL3 system register and other state. It also
defines the 'cpu_context' data structure which will used for saving and
restoring execution context for a given security state. These functions
will allow runtime services like PSCI and Secure payload dispatcher to
implement logic for switching between the secure and non-secure states.

The save and restore functions follow AArch64 PCS and only use
caller-saved temporary registers.

Change-Id: I8ee3aaa061d3caaedb28ae2c5becb9a206b6fd74