ARM: dts: sun8i: Add watchdog device node for A83T

The A83T, like previous Allwinner SoCs, has a watchdog as part of its
timer block. Add a device node for it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>