dt-bindings: arm64: dts: mediatek: Add mt7986 series

MT7986 series is Mediatek's new 4-core SoC, which is mainly
for wifi-router application. The difference between mt7986a and mt7986b
is that some pins do not exist on mt7986b.
Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211122123222.8016-2-sam.shih@mediatek.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Documentation/devicetree/bindings/arm/mediatek.yaml

@@ -77,6 +77,14 @@ properties:
           - enum:
               - mediatek,mt7629-rfb
           - const: mediatek,mt7629
+      - items:
+          - enum:
+              - mediatek,mt7986a-rfb
+          - const: mediatek,mt7986a
+      - items:
+          - enum:
+              - mediatek,mt7986b-rfb
+          - const: mediatek,mt7986b
       - items:
           - enum:
               - mediatek,mt8127-moose