watchdog: add support for sl28cpld watchdog

Add support for the watchdog of the sl28cpld board management
controller. This is part of a multi-function device driver.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>