mmc: mediatek: add pad_tune0 support

from mt2701, the register of PAD_TUNE has been phased out,
while there is a new register of PAD_TUNE0

Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Tested-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>