ARM: dts: sun8i: r40: add watchdog device node

The R40 SoC has a watchdog like the one on A20, in the timer memory zone
(which is also the same on A20).

Add the device tree node for it.

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