mmc: sdhci-esdhc-imx: add runtime pm support

The root clock will be disabled in runtime pm to save power.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>