watchdog: imx2_wdt: use devm_clk_get()

Use devm_clk_get() to make cleanup paths more simple.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>