mmc: at91-mci: remove obsolete driver

The at91-mci driver is not needed anymore since the atmel-mci driver now
supports all Atmel devices.

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