SPM: Implement global response buffer helpers

This buffer is where all the responses from Secure Partitions are stored
until they are requested.

Change-Id: Iafeb8f0848c5ff6f3e187060cd3a47702484dc45
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/services/std_svc/spm/spm.mk b/services/std_svc/spm/spm.mk
index a191f6f..b52292d 100644
--- a/services/std_svc/spm/spm.mk
+++ b/services/std_svc/spm/spm.mk
@@ -19,6 +19,7 @@
 			sp_setup.c				\
 			sp_xlat.c				\
 			spci.c					\
+			spm_buffers.c				\
 			spm_main.c				\
 			sprt.c)					\
 			${SPRT_LIB_SOURCES}