Documentation: dt: Add Kona PWM binding

Add the binding description for the Kona PWM controller found on Broadcom's
mobile SoCs.
Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/bcm-kona-pwm.txt

+Broadcom Kona PWM controller device tree bindings
+
+This controller has 6 channels.
+
+Required Properties :
+- compatible: should contain "brcm,kona-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@3e01a000 {
+	compatible = "brcm,bcm11351-pwm", "brcm,kona-pwm";
+	reg = <0x3e01a000 0xc4>;
+	clocks = <&pwm_clk>;
+	#pwm-cells = <3>;
+};