ATMEL: use generic mmc framework

gen_atmel_mci works on AVR32 as well, so no need to use the legacy
mmc driver. This also has the nice side effect of being able to use
SDHC cards an those boards.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>