pwm: add DT bindings documentation for atmel-hlcdc-pwm driver

The HLCDC IP available in some Atmel SoCs (i.e. at91sam9x5, at91sam9n12
or sama5d3 families for instance) provides a PWM device.

The DT bindings used for this PWM device is following the default 3 cells
bindings described in Documentation/devicetree/bindings/pwm/pwm.txt.
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NThierry Reding <thierry.reding@gmail.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

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

+Device-Tree bindings for Atmel's HLCDC (High-end LCD Controller) PWM driver
+
+The Atmel HLCDC PWM is subdevice of the HLCDC MFD device.
+See ../mfd/atmel-hlcdc.txt for more details.
+
+Required properties:
+ - compatible: value should be one of the following:
+   "atmel,hlcdc-pwm"
+ - pinctr-names: the pin control state names. Should contain "default".
+ - pinctrl-0: should contain the pinctrl states described by pinctrl
+   default.
+ - #pwm-cells: should be set to 3. This PWM chip use the default 3 cells
+   bindings defined in pwm.txt in this directory.
+
+Example:
+
+	hlcdc: hlcdc@f0030000 {
+		compatible = "atmel,sama5d3-hlcdc";
+		reg = <0xf0030000 0x2000>;
+		clocks = <&lcdc_clk>, <&lcdck>, <&clk32k>;
+		clock-names = "periph_clk","sys_clk", "slow_clk";
+
+		hlcdc_pwm: hlcdc-pwm {
+			compatible = "atmel,hlcdc-pwm";
+			pinctrl-names = "default";
+			pinctrl-0 = <&pinctrl_lcd_pwm>;
+			#pwm-cells = <3>;
+		};
+	};