watchdog: Add MOXA ART watchdog driver

This patch adds a watchdog driver for the main hardware watchdog timer
found on MOXA ART SoCs.

The MOXA ART SoC provides one writable timer register, restarting
the hardware once it reaches zero. The register is auto decremented
every APB clock cycle.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>