dt-bindings: pwm: tiehrpwm: Do not require pwm-cells twice

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

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Author: krzk

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

@@ -48,7 +48,6 @@ properties:
 required:
   - compatible
   - reg
-  - "#pwm-cells"
   - clocks
   - clock-names