Documentation: Add device tree bindings for Freescale FTM PWM.

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>;

};