dt-bindings: pwm: Add bindings doc for ZTE ZX PWM controller

ZTE ZX PWM controller

Required properties:

 - compatible: Should be "zte,zx296718-pwm".

 - reg: Physical base address and length of the controller's registers.

 - clocks : The phandle and specifier referencing the controller's clocks.

 - clock-names: "pclk" for PCLK, "wclk" for WCLK to the PWM controller.  The

   PCLK is for register access, while WCLK is the reference clock for

   calculating period and duty cycles.

 - #pwm-cells: Should be 3. See pwm.txt in this directory for a description of

   the cells format.

Example:

	pwm: pwm@1439000 {

		compatible = "zte,zx296718-pwm";

		reg = <0x1439000 0x1000>;

		clocks = <&lsp1crm LSP1_PWM_PCLK>,

			 <&lsp1crm LSP1_PWM_WCLK>;

		clock-names = "pclk", "wclk";

		#pwm-cells = <3>;

	};