feat(arm): add a macro for SPMC manifest base address

In RESET_TO_BL31, the SPMC manifest base address that is utilized by
bl32_image_ep_info has to be statically defined as DT is not available.
Common arm code sets this to the top of SRAM using macros but it can be
different for some platforms. Hence, introduce the macro
PLAT_ARM_SPMC_MANIFEST_BASE that could be re-defined by platform as per their
use-case. Platforms that utilize arm_def.h would use the existing value from
arm common code.

Signed-off-by: Rakshit Goyal <rakshit.goyal@arm.com>
Change-Id: I4491749ad2b5794e06c9bd11ff61e2e64f21a948