dt-bindings: display: advantech,idk-2121wr: reference common panel

Reference common panel bindings to bring descriptions of common fields
like panel-timing.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230823081107.82967-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>