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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>