mmc: dw_mmc: add runtime PM callback

This patch add dw_mci_runtime_suspend/resume interfaces
and expose it to dw_mci variant driver to support runtime
PM.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>