pwm: Add Berlin PWM controller DT binding

Add DT binding documentation for the Berlin PWM controller.
Signed-off-by: NAntoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: NSebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

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

+Berlin PWM controller
+
+Required properties:
+- compatible: should be "marvell,berlin-pwm"
+- reg: physical base address and length of the controller's registers
+- clocks: phandle to the input clock
+- #pwm-cells: should be 3. See pwm.txt in this directory for a description of
+  the cells format.
+
+Example:
+
+pwm: pwm@f7f20000 {
+	compatible = "marvell,berlin-pwm";
+	reg = <0xf7f20000 0x40>;
+	clocks = <&chip_clk CLKID_CFG>;
+	#pwm-cells = <3>;
+}