dt-bindings: pwm: mediatek,mt2712: Do not require pwm-cells twice

pwm-cells property is already required by pwm.yaml schema.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/0a0c13d6d716a52540e165e4835c0c406cc5d8ec.1714450308.git.zhoubinbin@loongson.cn


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