dt-bindings: pwm: Add Broadcom iProc PWM controller binding

Add a device tree binding for Broadcom iProc PWM controller.
Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/brcm,iproc-pwm.txt

+Broadcom iProc PWM controller device tree bindings
+
+This controller has 4 channels.
+
+Required Properties :
+- compatible: must be "brcm,iproc-pwm"
+- reg: physical base address and length of the controller's registers
+- clocks: phandle + clock specifier pair for the external clock
+- #pwm-cells: Should be 3. See pwm.txt in this directory for a
+  description of the cells format.
+
+Refer to clocks/clock-bindings.txt for generic clock consumer properties.
+
+Example:
+
+pwm: pwm@18031000 {
+	compatible = "brcm,iproc-pwm";
+	reg = <0x18031000 0x28>;
+	clocks = <&osc>;
+	#pwm-cells = <3>;
+};