RPi: Add myself as board maintainer

Commit 958d55f26ce ("MAINTAINERS: Take over BCM2835 maintainership") put
me in as maintainer for the RPi soc, but forgot to update the board
MAINTAINERS file.

Add me there too.

Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/board/raspberrypi/rpi/MAINTAINERS b/board/raspberrypi/rpi/MAINTAINERS
index a7eb365..cce1a7a 100644
--- a/board/raspberrypi/rpi/MAINTAINERS
+++ b/board/raspberrypi/rpi/MAINTAINERS
@@ -1,6 +1,6 @@
 RPI BOARD
-#M:	Stephen Warren <swarren@wwwdotorg.org>
-S:	Orphaned (Since 2017-07)
+M:	Alexander Graf <agraf@suse.de>
+S:	Maintained
 F:	board/raspberrypi/rpi/
 F:	include/configs/rpi.h
 F:	configs/rpi_*defconfig