MAINTAINERS: Add maintainer for the mmc subsystem

Put myself as a (co)maintainer of the mmc subsystem to help out Chris
Ball in a more official role.
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>
Signed-off-by: NChris Ball <chris@printf.net>

MAINTAINERS

@@ -5800,6 +5800,7 @@ F:	include/linux/mfd/
 
 MULTIMEDIA CARD (MMC), SECURE DIGITAL (SD) AND SDIO SUBSYSTEM
 M:	Chris Ball <chris@printf.net>
+M:	Ulf Hansson <ulf.hansson@linaro.org>
 L:	linux-mmc@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/cjb/mmc.git
 S:	Maintained