MAINTAINERS: hand over atmel-mci (sd/mmc interface)

Modify MAINTAINERS entry for Atmel SD/MMC drivers.
I hand the atmel-mci and at91_mci drivers over to Ludovic.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>