feat(el3-spmc): add a flag to enable support to load SEL0 SP

Introduce a build flag for enabling the support for loading SEL0 SP in
EL3 SPMC.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I1d63ae4d0d8374a732113565be90d58861506e39
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index f31365a..ea22655 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -240,6 +240,9 @@
 # Use the FF-A SPMC implementation in EL3.
 SPMC_AT_EL3			:= 0
 
+# Enable SEL0 SP when SPMC is enabled at EL3
+SPMC_AT_EL3_SEL0_SP		:=0
+
 # Use SPM at S-EL2 as a default config for SPMD
 SPMD_SPM_AT_SEL2		:= 1