mmc: s3cmci: Move to clk_prepare_enable/clk_disable_unprepare

Use clk_prepare_enable/clk_disable_unprepare to make the driver
work properly with common clock framework.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>