mmc: mediatek: add support of mt2701/mt2712

mt2701/mt2712 has 12bit clock div, which is not compatible with
mt8135/mt8173. and, some additional features will be added in
mt2701/mt2712, so that need distinguish it by comatibale name.

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>