dt-bindings: pwm: bcm2835: 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/85d7dc606d60a7d0d1557b98ac3a600c660b7421.1714450308.git.zhoubinbin@loongson.cnSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Documentation/devicetree/bindings/pwm/pwm-bcm2835.yaml

@@ -29,7 +29,6 @@ required:
   - compatible
   - reg
   - clocks
-  - "#pwm-cells"
 
 additionalProperties: false