pwm: imx-pwm: add explicit compatible strings and required clock properties

The imx-pwm binding contains language indicating compatible strings to
be used that is not valid for all supported parts e.g. Should be
"fsl,<soc>-pwm". Fix this by enumerating the valid compatible strings
that represent the two versions of this peripheral in use.

The binding is also missing the clocks/clock-names properties so
document these, the two required ipg and per clocks, and add these
properties to the example.

Signed-off-by: Matt Porter <mporter@konsulko.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

@@ -1,10 +1,17 @@
 Freescale i.MX PWM controller
 
 Required properties:
-- compatible: should be "fsl,<soc>-pwm"
+- compatible : should be "fsl,<soc>-pwm" and one of the following
+   compatible strings:
+  - "fsl,imx1-pwm" for PWM compatible with the one integrated on i.MX1
+  - "fsl,imx27-pwm" for PWM compatible with the one integrated on i.MX27
 - reg: physical base address and length of the controller's registers
 - #pwm-cells: should be 2. See pwm.txt in this directory for a description of
   the cells format.
+- clocks : Clock specifiers for both ipg and per clocks.
+- clock-names : Clock names should include both "ipg" and "per"
+See the clock consumer binding,
+	Documentation/devicetree/bindings/clock/clock-bindings.txt
 - interrupts: The interrupt for the pwm controller
 
 Example:
@@ -13,5 +20,8 @@ pwm1: pwm@53fb4000 {
 	#pwm-cells = <2>;
 	compatible = "fsl,imx53-pwm", "fsl,imx27-pwm";
 	reg = <0x53fb4000 0x4000>;
+	clocks = <&clks IMX5_CLK_PWM1_IPG_GATE>,
+		 <&clks IMX5_CLK_PWM1_HF_GATE>;
+	clock-names = "ipg", "per";
 	interrupts = <61>;
 };