watchdog: sunxi: New watchdog driver for Allwinner A10/A13

This patch adds the driver for the watchdog found in the Allwinner A10 and
A13 SoCs. It has DT-support and uses the new watchdog framework.

Signed-off-by: Carlo Caione <carlo.caione@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>