renesas: beacon-rzg2m: Add Marek to MAINTAINER file

Since any changes to the RZ/G2 family go through Marek's tree,
update the MAINTAINER file to automatically show his name
when running get_maintainer.pl.  Without this, he is not
copied.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>

board/beacon/beacon-rzg2m/MAINTAINERS

 BEACON_RZG2M BOARD
 M:	Adam Ford <aford173@gmail.com>
+M:	Marek Vasut <marek.vasut+renesas@mailbox.org>
 S:	Maintained
 F:	board/beacon/beacon-rzg2m/
 F:	include/configs/beacon-rzg2m.h