arm64: dts: Add Mediatek X20 Development Board support

Add initial device tree support for Mediatek X20 Development Board
based on MT6797 Deca core SoC. This board is one of the 96Boards
Consumer Edition platform.

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