dt-bindings: watchdog: add Allwinner H6 watchdog

Allwinner H6 has a similar watchdog as the A64 which is already
a compatible of the A31.

This commit add the H6 compatible.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>