dt-bindings: usb: mtk-xhci: add compatible for MT7988

MT7988 SoC contains two on-SoC XHCI controllers. Add proper binding.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240213130044.1976-1-zajec5@gmail.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/usb/mediatek,mtk-xhci.yaml

@@ -29,6 +29,7 @@ properties:
           - mediatek,mt7623-xhci
           - mediatek,mt7629-xhci
           - mediatek,mt7986-xhci
+          - mediatek,mt7988-xhci
           - mediatek,mt8173-xhci
           - mediatek,mt8183-xhci
           - mediatek,mt8186-xhci