ARM platforms: Allow board specific definition of SP stack base

The SGI platforms need to allocate memory for CPER buffers. These
platform buffers would be placed between the shared reserved memory
and the per cpu stack memory, thus the need to redefine stack base
pointer for these platforms. This patch allows each board in ARM
platform to define the PLAT_SP_IMAGE_STACK_BASE.

Change-Id: Ib5465448b860ab7ab0f645f7cb278a67acce7be9
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>