arm64: defconfig: Allow Mediatek boards to boot from the mmc

Enable the MMC_MTK option necessary to boot Mediatek boards from the
mmc. Booting from the mmc is the common method used for a bunch of
boards, specially Chromebooks, to boot from. Hence this driver is
built-in.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210423075250.2616275-1-enric.balletbo@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>