arm64: dts: allwinner: h6: add r_watchog node

Allwinner H6 has a r_watchdog similar to A64.

Declare it in the device-tree.

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