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])
  - low_threshold_brightness: brightness threshold low level. (get linear
		 scales in brightness in low end of brightness levels)

[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>;
	};

Example for brightness_threshold_level:

	backlight {
		compatible	= "pwm-backlight";
		pwms = <&pwm 0 50000>;

		brightness-levels = <0 4 8 16 32 64 128 255>;
		default-brightness-level = <6>;
		low_threshold_brightness = <50>;
	};
};
Note:
Low threshold support is required to have linear brightness scale from
0 to max. For some panels, backlight absent on low end of brightness
scale. So support for Low Threshold been required. So that the scale of
brightness changed from Low Threshold to Max in scales defined in
brightness-levels. In this example 20% maximum brightness scale should
be required to turn on panel backlight.
