mmc: mediatek: add support of source_cg clock

source clock need an independent cg to control, when doing clk mode
switch, need gate source clock to avoid hw issue(multi-bit sync hw hang)

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>