of: Add Atmel PWM controller device tree binding

The Atmel PWM controller uses the standard device tree bindings for PWM
controllers.
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

+Atmel PWM controller
+
+Required properties:
+  - compatible: should be one of:
+    - "atmel,at91sam9rl-pwm"
+    - "atmel,sama5d3-pwm"
+  - reg: physical base address and length of the controller's registers
+  - #pwm-cells: Should be 3. See pwm.txt in this directory for a
+    description of the cells format.
+
+Example:
+
+	pwm0: pwm@f8034000 {
+		compatible = "atmel,at91sam9rl-pwm";
+		reg = <0xf8034000 0x400>;
+		#pwm-cells = <3>;
+	};
+
+	pwmleds {
+		compatible = "pwm-leds";
+
+		d1 {
+			label = "d1";
+			pwms = <&pwm0 3 5000 0>
+			max-brightness = <255>;
+		};
+
+		d2 {
+			label = "d2";
+			pwms = <&pwm0 1 5000 1>
+			max-brightness = <255>;
+		};
+	};