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

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

@@ -66,7 +66,6 @@ properties:
 required:
   - compatible
   - reg
-  - "#pwm-cells"
   - clocks
   - clock-names