feat: support Cactus SP to boot on EL3 SPMC

EL3 SPMC has a limited set of FF-A functionality and does not support
all the features built into the current Cactus SP implementation.

Hence, this patch introduces a build option to strip out few features
from Cactus SP in order to boot it on EL3 SPMC.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I6319563adca67460015b79219ebc5c9468e3d54a
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index d26ec06..3a7f8bf 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -55,3 +55,7 @@
 # Use the Firmware Handoff framework to receive configurations from preceding
 # bootloader.
 TRANSFER_LIST		:= 0
+
+# This flag is required to match the feature set of Cactus SP that are
+# implemented in TF-A EL3 SPMC.
+SPMC_AT_EL3		:= 0