mmc: sdhci-s3c: Enable runtime power management

Since most of the work is already done by the core we just need to add
runtime suspend methods and tell the PM core that runtime PM is enabled
for this device.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>