pwm-backlight bindings

Required properties:
  - compatible: "pwm-backlight"
  - pwms: OF device-tree PWM specification (see PWM binding[0])
  - brightness-levels: Array of distinct brightness levels. Typically these
      are in the range from 0 to 255, but any range starting at 0 will do.
      The actual brightness level (PWM duty cycle) will be interpolated
      from these values. 0 means a 0% duty cycle (darkest/off), while the
      last value in the array represents a 100% duty cycle (brightest).
  - default-brightness-level: the default brightness level (index into the
      array defined by the "brightness-levels" property)

Optional properties:
  - pwm-names: a list of names for the PWM devices specified in the
               "pwms" property (see PWM binding[0])
  - enable-gpio: a GPIO used to enable the backlight.

Child device nodes describe alternate brightness-levels tables for different
display/backlight configurations.

[0]: Documentation/devicetree/bindings/pwm/pwm.txt

Example:

	backlight {
		compatible = "pwm-backlight";
		pwms = <&pwm 0 5000000>;

		brightness-levels = <0 4 8 16 32 64 128 255>;
		default-brightness-level = <6>;
		enable-gpio = <&gpx3 0 0>;
	};

Alternate brightness-levels tables

Child device nodes can specify multiple alternate brightness-levels tables,
identified by a generic string, and is selected upon calling the backlight
device's brightness_choose().

Properties:
- compatible: "pwm-backlight-alt-brightness-levels"
- identifier: A string that is used during backlight_choose() to select the
    particular alternate brightness-levels table.
- brightness-levels: See the brightness-levels property of the parent node,
    described earlier in this documentation.  Note that the length of these
    alternate brightness-level tables must be the same as the original
    brightness-levels table from the parent node.

Example:
	panel@0 {
		compatible = "pwm-backlight-alt-brightness-levels";
		identifier = "CMN-1132-1"
		brightness-levels = <0 16 32 40 80 120 160 255>;
	};