mmc: sdhi: Make runtime PM callbacks available for CONFIG_PM

To be able to simplify system PM, let's re-use the runtime PM callbacks
by converting to the SET_PM_RUNTIME_PM_OPS macro.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>