wdt: sunxi: Move restart code to the watchdog driver

Most of the watchdog code is duplicated between the machine restart code and
the watchdog driver. Add the restart hook to the watchdog driver, to be able to
remove it from the machine code eventually.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>