dt-bindings: arm64: mediatek: Add MT7988A and BPI-R4

MT7988A is another MediaTek's SoC with just 1 device available right
now: Banana Pi BPI-R4.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240108085228.4727-2-zajec5@gmail.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

Documentation/devicetree/bindings/arm/mediatek.yaml

@@ -95,6 +95,10 @@ properties:
           - enum:
               - mediatek,mt7986b-rfb
           - const: mediatek,mt7986b
+      - items:
+          - enum:
+              - bananapi,bpi-r4
+          - const: mediatek,mt7988a
       - items:
           - enum:
               - mediatek,mt8127-moose