dt-bindings: serial: mediatek,uart: add MT7981

Add compatible string for serial on MT7981 SoC.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240727121447.1016-1-zajec5@gmail.com


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