rtc: abx80x: add basic watchdog support

The abx804 and abx805 chips have support for a simple watchdog
function that can trigger an external reset.

Signed-off-by: Jeremy Gebben <jgebben@sweptlaser.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>