MAINTAINERS: add entry for Marvell Xenon MMC Host Controller drivers

Add maintainer entry for Marvell Xenon eMMC/SD/SDIO
Host Controller drivers.

Signed-off-by: Hu Ziji <huziji@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>