RPi: Add mbrugger as board maintainer

I took over maintainership from Alex Graf with commit
3157bbfa18 ("rpi: Make Matthias maintainer")
But I forgot to update the board maintainer file.
This patch adds myself to the game.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Alexander Graf <agraf@csgraf.de>
diff --git a/board/raspberrypi/rpi/MAINTAINERS b/board/raspberrypi/rpi/MAINTAINERS
index cce1a7a..4f1b23e 100644
--- a/board/raspberrypi/rpi/MAINTAINERS
+++ b/board/raspberrypi/rpi/MAINTAINERS
@@ -1,5 +1,5 @@
 RPI BOARD
-M:	Alexander Graf <agraf@suse.de>
+M:	Matthias Brugger <mbrugger@suse.com>
 S:	Maintained
 F:	board/raspberrypi/rpi/
 F:	include/configs/rpi.h