dt-bindings: pwm: mediatek,mt2712: add compatible for MT7988

MT7988 has on-SoC controller that can control up to 8 PWM interfaces. It
differs from blocks on other SoCs (amount of PWMs & registers) so it
needs its own compatible string.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240214140454.6438-1-zajec5@gmail.comSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Documentation/devicetree/bindings/pwm/mediatek,mt2712-pwm.yaml

@@ -24,6 +24,7 @@ properties:
           - mediatek,mt7629-pwm
           - mediatek,mt7981-pwm
           - mediatek,mt7986-pwm
+          - mediatek,mt7988-pwm
           - mediatek,mt8183-pwm
           - mediatek,mt8365-pwm
           - mediatek,mt8516-pwm