mmc: dw_mmc-rockchip: add runtime PM support

This patch adds runtime PM support for dw_mmc-rockchip.

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>