mmc: tmio: Keep host active while serving requests

Use runtime PM to keep the host active during I/O operations and other
requests which requires the tmio hardware to be powered.

Additionally make use of the runtime PM autosuspend feature with a
default timeout of 50 ms.

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