arm64: dts: allwinner: h6: add watchdog node

Allwinner H6 has a watchog node which seems broken
on some boards.

Test has been performed on several boards.

Chen-Yu Tsai boards:
Pine H64 - H6448BA 7782 => OK
OrangePi Lite 2 - H8068BA 61C2 => KO

Martin Ayotte boards:
Pine H64 - H8069BA 6892 => OK
OrangePi 3 - HA047BA 69W2 => KO
OrangePi One Plus - H7310BA 6842 => KO
OrangePi Lite2 - H6448BA 6662 => KO

Clément Péron board:
Beelink GS1 - H7309BA 6842 => KO

As it seems not fixable for now, declare the node
but leave it disable with a comment.

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