watchdog: menf21bmc_wdt: Introduce MEN 14F021P00 BMC Watchdog driver

Added driver to support the 14F021P00 BMC Watchdog.
The BMC is a Board Management Controller including watchdog functionality.

Signed-off-by: Andreas Werner <andreas.werner@men.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>