feat(mhu): use compile flag to choose mhu version

MHUv3 and MHUv2 drivers can now be selected at build time by using
PLAT_MHU_VERSION.

Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: I24f9e05f7969ed3be8f3261fdfed881a4ad18ba4

drivers/arm/rss/rss_comms.mk

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2022, Arm Limited. All rights reserved.
+# Copyright (c) 2022-2024, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -13,10 +13,19 @@ RSS_COMMS_SOURCES	:=	$(addprefix drivers/arm/rss/,			\
 					rss_comms_protocol_pointer_access.c	\
 				)
 
+ifeq (${PLAT_MHU_VERSION}, 3)
+RSS_COMMS_SOURCES	+=	$(addprefix drivers/arm/mhu/,			\
+					mhu_v3_x.c				\
+					mhu_wrapper_v3_x.c			\
+				)
+else ifeq (${PLAT_MHU_VERSION}, 2)
 RSS_COMMS_SOURCES	+=	$(addprefix drivers/arm/mhu/,			\
 					mhu_v2_x.c				\
 					mhu_wrapper_v2_x.c			\
 				)
+else
+$(error Unsupported MHU version)
+endif
 
 PLAT_INCLUDES		+=	-Idrivers/arm/rss		\
 				-Idrivers/arm/mhu