dt-bindings: pwm: mediatek: Add mediatek,mt7981 compatible

Add compatible string for the PWM unit found of the MediaTek MT7981 SoC.
This is in preparation to adding support in the pwm-mediatek.c driver.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>