mmc: dw_mmc-pltfm: deploy runtime PM facilities

Let's migrate it to use runtime PM and remove the system
PM callback from this driver. With this patch, it could
handle system PM properly and could also use runtime PM
if we enable it.

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>