AArch32: Fix SCTLR context initialization ...

This patch fixes a bug in context management library when writing
SCTLR register during context initialization. The write happened
prior to initialization of the register context pointer. This
resulted in the compiler optimizing the write sequence from the
final binary and hence SCTLR remains uninitialized when
entering normal world. The bug is fixed by doing the
initialization of the register context pointer earlier in the
sequence.

Change-Id: Ic7465593a74534046b79f40446ffa1165c52ed76

AArch32: Add support in TF libraries ...

This patch adds AArch32 support to cpu ops, context management,
per-cpu data and spinlock libraries. The `entrypoint_info`
structure is modified to add support for AArch32 register
arguments. The CPU operations for AEM generic cpu in AArch32
mode is also added.

Change-Id: I1e52e79f498661d8f31f1e7b3a29e222bc7a4483

Introduce `el3_runtime` and `PSCI` libraries ...

This patch moves the PSCI services and BL31 frameworks like context
management and per-cpu data into new library components `PSCI` and
`el3_runtime` respectively. This enables PSCI to be built independently from
BL31. A new `psci_lib.mk` makefile is introduced which adds the relevant
PSCI library sources and gets included by `bl31.mk`. Other changes which
are done as part of this patch are:

* The runtime services framework is now moved to the `common/` folder to
  enable reuse.
* The `asm_macros.S` and `assert_macros.S` helpers are moved to architecture
  specific folder.
* The `plat_psci_common.c` is moved from the `plat/common/aarch64/` folder
  to `plat/common` folder. The original file location now has a stub which
  just includes the file from new location to maintain platform compatibility.

Most of the changes wouldn't affect platform builds as they just involve
changes to the generic bl1.mk and bl31.mk makefiles.

NOTE: THE `plat_psci_common.c` FILE HAS MOVED LOCATION AND THE STUB FILE AT
THE ORIGINAL LOCATION IS NOW DEPRECATED. PLATFORMS SHOULD MODIFY THEIR
MAKEFILES TO INCLUDE THE FILE FROM THE NEW LOCATION.

Change-Id: I6bd87d5b59424995c6a65ef8076d4fda91ad5e86