dt-bindings: watchdog: meson-gxbb-wdt: Include generic watchdog bindings

Include generic watchdog DT schema bindings in Amlogic GXBB Watchdog
bindings.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>