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>
diff --git a/drivers/pwm/pwm-mxs.c b/drivers/pwm/pwm-mxs.c
index 9602708..d9a3805 100644
--- a/drivers/pwm/pwm-mxs.c
+++ b/drivers/pwm/pwm-mxs.c
@@ -186,7 +186,7 @@
 }
 
 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);