ARM: meson: add watchdog driver

This patch adds the watchdog driver for the Amlogic Meson SoCs used also
to reboot the device.

Signed-off-by: Carlo Caione <carlo@caione.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>