mmc: mtk-sd: add support for config found in mt7620 family SOCs.

mt7620 family MIPS SOCs contain the mtk-sd silicon.
Add support for this.

Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>