mmc: mediatek: add ops->get_cd() support

if user plug out sd card slowly, finally card is plugged out but
cat /proc/partitions can find that card is still exist in kernel.
that's because alougth get card detect interrupt but CMD13 still
can get correct response(all other pins are connected expect card
detect pin).
add ops->get_cd() can avoid this issue.

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