media: dt-bindings: amlogic,meson-gx-ao-cec: reference common CEC properties

Reference common HDMI CEC adapter properties to simplify the binding and
have only one place of definition for common properties.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>