feat(spmc): enable building of the SPMC at EL3

Introduce build flag for enabling the secure partition
manager core, SPMC_AT_EL3. When enabled, the SPMC module
will be included into the BL31 image. By default the
flag is disabled.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I5ea1b953e5880a07ffc91c4dea876a375850cf2a
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 99f44a4..7b66569 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -288,6 +288,9 @@
 # Enable the Management Mode (MM)-based Secure Partition Manager implementation
 SPM_MM				:= 0
 
+# Use the FF-A SPMC implementation in EL3.
+SPMC_AT_EL3			:= 0
+
 # Use SPM at S-EL2 as a default config for SPMD
 SPMD_SPM_AT_SEL2		:= 1