watchdog: mena21_wdt: Convert to GPIO descriptors

This drops the old OF API use to look up global GPIO
numbers and replace it with the GPIO descriptor API.

Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Johannes Thumshirn <morbidrsa@gmail.com>
Cc: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>