mmc: sdhci-s3c: Use CONFIG_PM_SLEEP to ifdef system suspend

This matches current best practice as one can have runtime PM enabled
without system sleep and CONFIG_PM is defined for both.

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>