Spreadtrum backlight bindings

Required properties:
 - compatible: "sprd,sharkl5pro-backlight"
 - pwms: OF device-tree PWM specification (see PWM binding[0])
 - pwm-names: a list of names for the PWM devices specified in the
     "pwms" property (see PWM binding[0])
 - sprd,max-brightness-level: The max brightness level.
     Between min brightness level and max brightness level
     represents the actual brightness levels array
     (min brightness level defined by the "sprd,min-brightness-level" property)
 - default-brightness-level: the default brightness level.
     Must take a value between min brightness level and max brightness level
     see pwm-backlight binding[1]
 - sprd,brightness-scale: This value represents a 100% duty cycle (brightest)

Optional properties:
  - sprd,min-brightness-level: The min brightness level, default is 0.

[0]: Documentation/devicetree/bindings/pwm/pwm.txt
[1]: Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt

Example:

	backlight {
		compatible = "sprd,sharkl5pro-backlight";
		pwms = <&pwm 2 40000>;
		pwm-names = "backlight";
		sprd,max-brightness-level = <240>;
		default-brightness-level = <33>;
		sprd,brightness-scale = <255>;
	};
