dt-bindings: watchdog: meson-gxbb-wdt: convert to yaml

Now that we have the DT validation in place, let's convert the device tree
bindings for the Amlogic GXBB Watchdog timer over to a YAML schemas.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>