watchdog: imx2_wdt: Add power management support.

Add power management operations(suspend and resume) as part of
dev_pm_ops for IMX2 watchdog driver.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>