dt-bindings: arm: mediatek: Document Mediatek X20 Development Board

Document Mediatek X20 Development Board which is a 96Boards Consumer
Edition platform based on MT6797 SoC.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>