dt-bindings: pwm: imx-pwm: Unify #pwm-cells for all compatibles

Use #pwm-cells for all i.MX variants. Only fsl,imx1-pwm does not support
inverted PWM output. Keep it the same for consistency.
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

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

@@ -14,12 +14,10 @@ allOf:
 
 properties:
   "#pwm-cells":
-    description: |
-      Should be 2 for i.MX1 and 3 for i.MX27 and newer SoCs. See pwm.yaml
-      in this directory for a description of the cells format.
-    enum:
-      - 2
-      - 3
+    description:
+      The only third cell flag supported by this binding is
+      PWM_POLARITY_INVERTED. fsl,imx1-pwm does not support this flags.
+    const: 3
 
   compatible:
     oneOf: