plat/arm: allow RESET_TO_BL31 for CSS-based platforms

This lets any future CSS platforms to use RESET_TO_BL31 flag.

Change-Id: I32a90fce43cb0c6f4d33589653a0fd6a7ecc9577
Signed-off-by: Deepak Pandey <Deepak.Pandey@arm.com>
diff --git a/plat/arm/css/sgi/sgi-common.mk b/plat/arm/css/sgi/sgi-common.mk
index 28f97b1..24f03dd 100644
--- a/plat/arm/css/sgi/sgi-common.mk
+++ b/plat/arm/css/sgi/sgi-common.mk
@@ -67,6 +67,11 @@
 # Add the HW_CONFIG to FIP and specify the same to certtool
 $(eval $(call TOOL_ADD_PAYLOAD,${HW_CONFIG},--hw-config))
 
+ifneq (${RESET_TO_BL31},0)
+  $(error "Using BL31 as the reset vector is not supported on ${PLATFORM} platform. \
+  Please set RESET_TO_BL31 to 0.")
+endif
+
 $(eval $(call add_define,SGI_PLAT))
 
 override CSS_LOAD_SCP_IMAGES	:=	0