build(rss): introduce rss_comms.mk makefile

Provide a new makefile as a convenience for platform makefiles to pull
in the list of source files and headers for the RSS communication
driver.

Change-Id: I188a1a8f4e77318cdc87c3155b280090c46ce813
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/drivers/arm/rss/rss_comms.mk b/drivers/arm/rss/rss_comms.mk
new file mode 100644
index 0000000..8f19a0b
--- /dev/null
+++ b/drivers/arm/rss/rss_comms.mk
@@ -0,0 +1,20 @@
+#
+# Copyright (c) 2022, Arm Limited. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+
+RSS_COMMS_SOURCES	:=	$(addprefix drivers/arm/rss/,			\
+					rss_comms.c				\
+					rss_comms_protocol.c			\
+					rss_comms_protocol_embed.c		\
+					rss_comms_protocol_pointer_access.c	\
+				)
+
+RSS_COMMS_SOURCES	+=	$(addprefix drivers/arm/mhu/,			\
+					mhu_v2_x.c				\
+					mhu_wrapper_v2_x.c			\
+				)
+
+PLAT_INCLUDES		+=	-Idrivers/arm/rss		\
+				-Idrivers/arm/mhu