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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Matthias Brugger <[e-mail]>

Documentation/devicetree/bindings/arm/mediatek.txt

···
 - Evaluation board for MT6797(Helio X20):
     Required root node properties:
       - compatible = "mediatek,mt6797-evb", "mediatek,mt6797";
+- Mediatek X20 Development Board:
+    Required root node properties:
+      - compatible = "archermind,mt6797-x20-dev", "mediatek,mt6797";
 - Reference board variant 1 for MT7622:
     Required root node properties:
       - compatible = "mediatek,mt7622-rfb1", "mediatek,mt7622";