MAINTAINERS: add myself as atmel-mci maintainer (sd/mmc interface)

Add MAINTAINERS entry for atmel-mci driver.
This driver was maintained by its author: Haavard Skinnemoen. I take the
maintainance of it.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Pierre Ossman <pierre@ossman.eu>