Documentation: Add device tree bindings for Freescale FTM PWM.

This adds the binding documentation for Freescale FlexTimer Module
(FTM) PWM driver under Documentation/devicetree/bindings/pwm/.
Signed-off-by: NXiubo Li <Li.Xiubo@freescale.com>
Reviewed-by: NSascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: NYuan Yao <yao.yuan@freescale.com>
Acked-by: NKumar Gala <galak@codeaurora.org>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-fsl-ftm.txt

+Freescale FlexTimer Module (FTM) PWM controller
+
+Required properties:
+- compatible: Should be "fsl,vf610-ftm-pwm".
+- reg: Physical base address and length of the controller's registers
+- #pwm-cells: Should be 3. See pwm.txt in this directory for a description of
+  the cells format.
+- clock-names: Should include the following module clock source entries:
+    "ftm_sys" (module clock, also can be used as counter clock),
+    "ftm_ext" (external counter clock),
+    "ftm_fix" (fixed counter clock),
+    "ftm_cnt_clk_en" (external and fixed counter clock enable/disable).
+- clocks: Must contain a phandle and clock specifier for each entry in
+  clock-names, please see clock/clock-bindings.txt for details of the property
+  values.
+- pinctrl-names: Must contain a "default" entry.
+- pinctrl-NNN: One property must exist for each entry in pinctrl-names.
+  See pinctrl/pinctrl-bindings.txt for details of the property values.
+
+
+Example:
+
+pwm0: pwm@40038000 {
+		compatible = "fsl,vf610-ftm-pwm";
+		reg = <0x40038000 0x1000>;
+		#pwm-cells = <3>;
+		clock-names = "ftm_sys", "ftm_ext",
+				"ftm_fix", "ftm_cnt_clk_en";
+		clocks = <&clks VF610_CLK_FTM0>,
+			<&clks VF610_CLK_FTM0_EXT_SEL>,
+			<&clks VF610_CLK_FTM0_FIX_SEL>,
+			<&clks VF610_CLK_FTM0_EXT_FIX_EN>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_pwm0_1>;
+};