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.com


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>