watchdog: s3c2410_wdt: use clk_prepare_enable and clk_disable_unprepare

Convert clk_enable/clk_disable to clk_prepare_enable/clk_disable_unprepare
calls as required by common clock framework.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>