mmc: tmio_mmc: Enable runtime PM support

To take advantage of the clock gating support, use the runtime PM
callbacks provided by the tmio core.

Additionally, we make use of the SET_PM_RUNTIME_PM_OPS, which is a
preparation needed to simplify system PM.

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