feat(plat/arm): allow BL32 specific defines to be used by SPMC_AT_EL3

For EL3 SPMC configuration enabled platforms, allow the reuse of
BL32 specific definitions.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I37ffbbf680326c101fbb2f146085a96c138f07a1