mmc: mediatek: add MT6779 MMC driver support

Add new code to support MT6779 mmc driver.

Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>
Link: https://lore.kernel.org/r/1595205759-5825-2-git-send-email-chun-hung.wu@mediatek.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>