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>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>