pwm: Add device tree binding document for IMG PWM DAC

Add binding document for IMG Pulse Width Modulator (PWM) DAC present on
the Pistachio SOC. The PWM DAC has four channels.
Signed-off-by: Naidu Tellapati <Naidu.Tellapati@imgtec.com>
Signed-off-by: Sai Masarapu <Sai.Masarapu@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

+*Imagination Technologies PWM DAC driver
+
+Required properties:
+  - compatible: Should be "img,pistachio-pwm"
+  - reg: Should contain physical base address and length of pwm registers.
+  - clocks: Must contain an entry for each entry in clock-names.
+	See ../clock/clock-bindings.txt for details.
+  - clock-names: Must include the following entries.
+    - pwm: PWM operating clock.
+    - sys: PWM system interface clock.
+  - #pwm-cells: Should be 2. See pwm.txt in this directory for the
+	description of the cells format.
+  - img,cr-periph: Must contain a phandle to the peripheral control
+	syscon node which contains PWM control registers.
+
+Example:
+	pwm: pwm@18101300 {
+		compatible = "img,pistachio-pwm";
+		reg = <0x18101300 0x100>;
+		clocks = <&pwm_clk>, <&system_clk>;
+		clock-names = "pwm", "sys";
+		#pwm-cells = <2>;
+		img,cr-periph = <&cr_periph>;
+	};