AArch32: Add essential ARM platform and FVP support ...

This patch adds AArch32 support for FVP and implements common platform APIs
like `plat_get_my_stack`, `plat_set_my_stack`, `plat_my_core_cos` for AArch32.
Only Multi Processor(MP) implementations of these functions are considered in
this patch. The ARM Standard platform layer helpers are implemented for
AArch32 and the common makefiles are modified to cater for both AArch64 and
AArch32 builds. Compatibility with the deprecated platform API is not
supported for AArch32.

Change-Id: Iad228400613eec91abf731b49e21a15bcf2833ea

Define `plat_get_syscnt_freq2()` unconditionally for ARM platforms ...

Previously the definition of `plat_get_syscnt_freq2()` in `arm_common.c` was
conditionally defined based on the ERROR_DEPRECATED flag. This patch makes
this function available irrespective of the flag and removes the deprecated
`plat_get_syscnt_freq()` definition.

Change-Id: I250ca787ca1b5e867096c6ba8f2bb444db44c97b

Cater for preloaded BL33 within plat_get_ns_image_entrypoint() ...

The PRELOADED_BL33_BASE build option allows to preload a BL33 and bypass its
loading by BL2. In ARM standard platforms, the conditional behaviour of
PRELOADED_BL33_BASE is moved within the implementation of
`plat_get_ns_image_entrypoint()` so that all callers may benefit from this
feature.

Change-Id: Iea060e204ec72f8081087837854535c4e320da4e

Move `arm_common.c` out of aarch64 folder ...

This patch moves the `arm_common.c` file from `plat/arm/common/aarch64/`
to the parent directory since the functions implemented in the file are
not AArch64 specific. The platform makefiles are also modified for this
change.

Change-Id: I776d2e4958f59041476cf2f53a9adb5b2d304ee0