feat(plat/fvp): introduce accessor function to obtain datastore

In order to provide the EL3 SPMC a sufficient datastore to
record memory descriptors, a accessor function is used.
This allows for the backing memory to be allocated in a
platform defined manner, to accommodate memory constraints
and desired use cases.

Provide an implementation for the Arm FVP platform to
use a default value of 512KB memory allocated in the
TZC RAM section.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I92bc55ba6e04bdad429eb52f0d2960ceda682804
diff --git a/plat/arm/board/fvp/platform.mk b/plat/arm/board/fvp/platform.mk
index 89ca185..54c5e75 100644
--- a/plat/arm/board/fvp/platform.mk
+++ b/plat/arm/board/fvp/platform.mk
@@ -425,3 +425,7 @@
 
 # enable trace filter control registers access to NS by default
 ENABLE_TRF_FOR_NS		:= 1
+
+ifeq (${SPMC_AT_EL3}, 1)
+PLAT_BL_COMMON_SOURCES	+=	plat/arm/board/fvp/fvp_el3_spmc.c
+endif