watchdog: mv64x60_wdt: use devm_ioremap()

Use devm_ioremap() to make cleanup paths simpler.

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>