dt-bindings: serial: mediatek,uart: add MT7988

Add compatible string for serial on MT7988 SoC.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240605085433.26513-1-zajec5@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/mediatek,uart.yaml

@@ -37,6 +37,7 @@ properties:
               - mediatek,mt7623-uart
               - mediatek,mt7629-uart
               - mediatek,mt7986-uart
+              - mediatek,mt7988-uart
               - mediatek,mt8127-uart
               - mediatek,mt8135-uart
               - mediatek,mt8173-uart