serial: s5p: use clock api to get clock rate

On Exynos platforms that support clock driver API, allow the driver to
use clock api get the SCLK clock rate.

Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>