avr32: Use CONFIG_ATMEL_MCI to select the atmel_mci driver

After we move the atmel_mci driver into drivers/mmc, we can't select
it with CONFIG_MMC anymore. Introduce a new symbol specifically for
this driver so that there's no ambiguity.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Acked-by: Jean-Chritophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>