dt-bindings: arm64: dts: mediatek: add compatible for bananapi r3

Add compatible for mt7986 based BananaPi R3 SBC.
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221106085034.12582-9-linux@fw-web.deSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Documentation/devicetree/bindings/arm/mediatek.yaml

@@ -83,6 +83,7 @@ properties:
           - const: mediatek,mt7629
       - items:
           - enum:
+              - bananapi,bpi-r3
               - mediatek,mt7986a-rfb
           - const: mediatek,mt7986a
       - items: