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/include/plat/arm/common/arm_spm_def.h b/include/plat/arm/common/arm_spm_def.h
index 5c7ca90..bf3cb8f 100644
--- a/include/plat/arm/common/arm_spm_def.h
+++ b/include/plat/arm/common/arm_spm_def.h
@@ -139,5 +139,6 @@
 #define PLAT_SPM_SERVICES_MAX		U(30)
 
 #define PLAT_SPCI_HANDLES_MAX_NUM	U(20)
+#define PLAT_SPM_RESPONSES_MAX		U(30)
 
 #endif /* ARM_SPM_DEF_H */