mmc: sunxi: Add runtime_pm support

So far, even if our card was not in use, we didn't shut down our MMC
controller, which meant that it was still active and clocking the bus.

While this obviously means that we could save some power there, it also
creates issues when it comes to EMC control since we'll have a perfect peak
at the card clock rate.

Let's implement runtime_pm with autosuspend so that we will shut down the
controller when it's not been in use for quite some time.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>