pwm: pwm-mxs: encode soc name in compatible string

Encode soc name in the compatible string to know the specific version
hardware block.  This is the general approach adopted for most bindings.
Change mxs-pwm binding to use the approach.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>

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

 Freescale MXS PWM controller
 
 Required properties:
-- compatible: should be "fsl,mxs-pwm"
+- compatible: should be "fsl,imx23-pwm"
 - reg: physical base address and length of the controller's registers
 - #pwm-cells: should be 2.  The first cell specifies the per-chip index
   of the PWM to use and the second cell is the duty cycle in nanoseconds.
@@ -10,7 +10,7 @@ Required properties:
 Example:
 
 pwm: pwm@80064000 {
-	compatible = "fsl,imx28-pwm", "fsl,mxs-pwm";
+	compatible = "fsl,imx28-pwm", "fsl,imx23-pwm";
 	reg = <0x80064000 2000>;
 	#pwm-cells = <2>;
 	fsl,pwm-number = <8>;

drivers/pwm/pwm-mxs.c

@@ -186,7 +186,7 @@ static int __devexit mxs_pwm_remove(struct platform_device *pdev)
 }
 
 static struct of_device_id mxs_pwm_dt_ids[] = {
-	{ .compatible = "fsl,mxs-pwm", },
+	{ .compatible = "fsl,imx23-pwm", },
 	{ /* sentinel */ }
 };
 MODULE_DEVICE_TABLE(of, mxs_pwm_dt_ids);