dt-bindings: pwm: sprd: Add Spreadtrum PWM documentation

Spreadtrum PWM controller

Spreadtrum SoCs PWM controller provides 4 PWM channels.

Required properties:

- compatible : Should be "sprd,ums512-pwm".

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

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

- clock-names: Should contain following entries:

  "pwmn": used to derive the functional clock for PWM channel n (n range: 0 ~ 3).

  "enablen": for PWM channel n enable clock (n range: 0 ~ 3).

- #pwm-cells: Should be 2. See pwm.txt in this directory for a description of

  the cells format.

Optional properties:

- assigned-clocks: Reference to the PWM clock entries.

- assigned-clock-parents: The phandle of the parent clock of PWM clock.

Example:

	pwms: pwm@32260000 {

		compatible = "sprd,ums512-pwm";

		reg = <0 0x32260000 0 0x10000>;

		clock-names = "pwm0", "enable0",

			"pwm1", "enable1",

			"pwm2", "enable2",

			"pwm3", "enable3";

		clocks = <&aon_clk CLK_PWM0>, <&aonapb_gate CLK_PWM0_EB>,

		       <&aon_clk CLK_PWM1>, <&aonapb_gate CLK_PWM1_EB>,

		       <&aon_clk CLK_PWM2>, <&aonapb_gate CLK_PWM2_EB>,

		       <&aon_clk CLK_PWM3>, <&aonapb_gate CLK_PWM3_EB>;

		assigned-clocks = <&aon_clk CLK_PWM0>,

			<&aon_clk CLK_PWM1>,

			<&aon_clk CLK_PWM2>,

			<&aon_clk CLK_PWM3>;

		assigned-clock-parents = <&ext_26m>,

			<&ext_26m>,

			<&ext_26m>,

			<&ext_26m>;

		#pwm-cells = <2>;

	};