fix(mhu): use MHUv2 if PLAT_MHU_VERSION undefined

If RSS Comms is used but PLAT_MHU_VERSION was undefined then it should
default to MHUv2 to avoid breaking existing configurations which did not
need to specify PLAT_MHU_VERSION as on MHUv2 was available.

Change-Id: I8353b49b9f61414a664c2802f90ba3b2bc526887
Signed-off-by: Joel Goddard <joel.goddard@arm.com>

drivers/arm/rss/rss_comms.mk

@@ -13,6 +13,9 @@ RSS_COMMS_SOURCES	:=	$(addprefix drivers/arm/rss/,			\
 					rss_comms_protocol_pointer_access.c	\
 				)
 
+# Default to MHUv2 if PLAT_MHU_VERSION undefined
+PLAT_MHU_VERSION ?= 2
+
 ifeq (${PLAT_MHU_VERSION}, 3)
 RSS_COMMS_SOURCES	+=	$(addprefix drivers/arm/mhu/,			\
 					mhu_v3_x.c				\