mmc: mediatek: add stop_clk fix and enhance_rx support

mt2712 supports stop_clk fix and enhance_rx, which can improve
host stability.

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>