ARM: rmobile: Add missing rcar-common/common.c to Beacon RZG2M kit

The rcar-common/common.c contains various common board functions shared
by all R-Car and RZG boards. This board is not compiling the file in, so
add it. This way, part of the board code can be de-duplicated too.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Adam Ford <aford173@gmail.com>
diff --git a/board/beacon/beacon-rzg2m/Makefile b/board/beacon/beacon-rzg2m/Makefile
index 439f319..9131a92 100644
--- a/board/beacon/beacon-rzg2m/Makefile
+++ b/board/beacon/beacon-rzg2m/Makefile
@@ -6,4 +6,4 @@
 # SPDX-License-Identifier: GPL-2.0+
 #
 
-obj-y	:= beacon-rzg2m.o
+obj-y	:= beacon-rzg2m.o ../../renesas/rcar-common/common.o