dt-bindings: pwm: sifive: Include generic PWM schema

Include generic pwm.yaml schema, which enforces PWM node naming and
brings pwm-cells requirement.

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-sifive.yaml

@@ -22,6 +22,9 @@ description:
 
   https://github.com/sifive/sifive-blocks/tree/master/src/main/scala/devices/pwm
 
+allOf:
+  - $ref: pwm.yaml#
+
 properties:
   compatible:
     items:
@@ -55,7 +58,6 @@ required:
   - compatible
   - reg
   - clocks
-  - "#pwm-cells"
   - interrupts
 
 additionalProperties: false