watchdog: dw_wdt: use clk_prepare_enable and clk_disable_unprepare

This is necessary to make the driver work with platforms using the
common clock framework.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>