dt-bindings: pwm: mxs: Document fsl,imx28-pwm

imx28 uses the same PWM block that is found on imx23.

Add an entry for fsl,imx28-pwm.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

@@ -15,8 +15,12 @@ allOf:
 
 properties:
   compatible:
-    enum:
-      - fsl,imx23-pwm
+    oneOf:
+      - const: fsl,imx23-pwm
+      - items:
+          - enum:
+              - fsl,imx28-pwm
+          - const: fsl,imx23-pwm
 
   reg:
     maxItems: 1