arm64: dts: allwinner: a64: Add watchdog

Add a watchdog node for the A64, automatically enabled on all boards.
Since the device is compatible with an existing driver, we only reserve
a new compatible string to be used together with the fall back.
Tested on Olimex Teres-I.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>