watchdog: s3c2410_wdt: convert s3c2410wdt to dev_pm_ops

Instead of using legacy suspend/resume methods, using newer
dev_pm_ops structure allows better control over power management.

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>