watchdog: dw_wdt: convert to SIMPLE_DEV_PM_OPS

The dw_wdt only provides PM_SLEEP operations, so convert the driver
to use SIMPLE_DEV_PM_OPS instead of populating the struct manually.
This has the added effect of simplifying the CONFIG_PM ifdefs.

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>