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

Add Spreadtrum PWM controller documentation.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

+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>;
+	};