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>