MAINTAINERS: Take over BCM2835 maintainership

It seems as if I have more interest in BCM2835 support than most others,
so I'll bite the bullet and declare myself maintainer. It'd be a shame
to leave that platform orphaned.

Signed-off-by: Alexander Graf <agraf@suse.de>

MAINTAINERS

@@ -92,8 +92,8 @@ T:	git git://git.denx.de/u-boot-atmel.git
 F:	arch/arm/mach-at91/
 
 ARM BROADCOM BCM283X
-#M:	Stephen Warren <swarren@wwwdotorg.org>
+M:	Alexander Graf <agraf@suse.de>
-S:	Orphaned (Since 2017-07)
+S:	Maintained
 F:	arch/arm/mach-bcm283x/
 F:	drivers/gpio/bcm2835_gpio.c
 F:	drivers/mmc/bcm2835_sdhci.c