This patch adds support for booting EL3 payloads on CSS platforms,
for example Juno. In this scenario, the Trusted Firmware follows
its normal boot flow up to the point where it would normally pass
control to the BL31 image. At this point, it jumps to the EL3
payload entry point address instead.

Before handing over to the EL3 payload, the data SCP writes for AP
at the beginning of the Trusted SRAM is restored, i.e. we zero the
first 128 bytes and restore the SCP Boot configuration. The latter
is saved before transferring the BL30 image to SCP and is restored
just after the transfer (in BL2). The goal is to make it appear that
the EL3 payload is the first piece of software to run on the target.

The BL31 entrypoint info structure is updated to make the primary
CPU jump to the EL3 payload instead of the BL31 image.

The mailbox is populated with the EL3 payload entrypoint address,
which releases the secondary CPUs out of their holding pen (if the
SCP has powered them on). The arm_program_trusted_mailbox() function
has been exported for this purpose.

The TZC-400 configuration in BL2 is simplified: it grants secure
access only to the whole DRAM. Other security initialization is
unchanged.

This alternative boot flow is disabled by default. A new build option
EL3_PAYLOAD_BASE has been introduced to enable it and provide the EL3
payload's entry point address. The build system has been modified
such that BL31 and BL33 are not compiled and/or not put in the FIP in
this case, as those images are not used in this boot flow.

Change-Id: Id2e26fa57988bbc32323a0effd022ab42f5b5077

The communication protocol used between the AP cores and the SCP
in CSS-based platforms like Juno has undergone a number of changes.
This patch makes the required modifications to the SCP Boot Protocol,
SCPI Protocol and MHU driver code in shared CSS platform code so that
the AP cores are still able to communicate with the SCP.

This patch focuses on the mandatory changes to make it work. The
design of this code needs to be improved but this will come in
a subsequent patch.

The main changes are:

 - MHU communication protocol

   - The command ID and payload size are no longer written into the
     MHU registers directly. Instead, they are stored in the payload
     area. The MHU registers are now used only as a doorbell to kick
     off messages. Same goes for any command result, the AP has to
     pick it up from the payload area.

 - SCP Boot Protocol

   - The BL3-0 image is now expected to embed a checksum. This
     checksum must be passed to the SCP, which uses it to check the
     integrity of the image it received.

   - The BL3-0 image used to be transferred a block (4KB)
     at a time. The SCP now supports receiving up to 128KB at a
     time, which is more than the size of the BL3-0 image.
     Therefore, the image is now sent in one go.

   - The command IDs have changed.

 - SCPI Protocol

   - The size of the SCPI payload has been reduced down from 512
     bytes to 256 bytes. This changes the base address of the
     AP-to-SCP payload area.

   - For commands that have a response, the response is the same SCPI
     header that was sent, except for the size and the status, which
     both must be updated appropriately. Success/Failure of a command
     is determined by looking at the updated status code.

   - Some command IDs have changed.

NOTE: THIS PATCH BREAKS COMPATIBILITY WITH FORMER VERSIONS OF THE SCP
FIRMWARE AND THUS REQUIRES AN UPDATE OF THIS BINARY. THE LATEST SCP
BINARY CAN BE OBTAINED FROM THE ARM CONNECTED COMMUNITY WEBSITE.

Change-Id: Ia5f6b95fe32401ee04a3805035748e8ef6718da7

This major change pulls out the common functionality from the
FVP and Juno platform ports into the following categories:

*   (include/)plat/common. Common platform porting functionality that
typically may be used by all platforms.

*   (include/)plat/arm/common. Common platform porting functionality
that may be used by all ARM standard platforms. This includes all
ARM development platforms like FVP and Juno but may also include
non-ARM-owned platforms.

*   (include/)plat/arm/board/common. Common platform porting
functionality for ARM development platforms at the board
(off SoC) level.

*   (include/)plat/arm/css/common. Common platform porting
functionality at the ARM Compute SubSystem (CSS) level. Juno
is an example of a CSS-based platform.

*   (include/)plat/arm/soc/common. Common platform porting
functionality at the ARM SoC level, which is not already defined
at the ARM CSS level.

No guarantees are made about the backward compatibility of
functionality provided in (include/)plat/arm.

Also remove any unnecessary variation between the ARM development
platform ports, including:

*   Unify the way BL2 passes `bl31_params_t` to BL3-1. Use the
Juno implementation, which copies the information from BL2 memory
instead of expecting it to persist in shared memory.

*   Unify the TZC configuration. There is no need to add a region
for SCP in Juno; it's enough to simply not allow any access to
this reserved region. Also set region 0 to provide no access by
default instead of assuming this is the case.

*   Unify the number of memory map regions required for ARM
development platforms, although the actual ranges mapped for each
platform may be different. For the FVP port, this reduces the
mapped peripheral address space.

These latter changes will only be observed when the platform ports
are migrated to use the new common platform code in subsequent
patches.

Change-Id: Id9c269dd3dc6e74533d0e5116fdd826d53946dc8