[MMC] Add maintainers entry for MMC subsystem

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

MAINTAINERS

@@ -1877,6 +1877,11 @@ L:	linux-kernel@vger.kernel.org
 W:	http://www.atnf.csiro.au/~rgooch/linux/kernel-patches.html
 S:	Maintained
 
+MULTIMEDIA CARD SUBSYSTEM
+P:	Russell King
+M:	rmk+mmc@arm.linux.org.uk
+S:	Maintained
+
 MULTISOUND SOUND DRIVER
 P:	Andrew Veliath
 M:	andrewtv@usa.net