lp88xx bindings

Required properties:
  - compatible: Should be one of "ti,lp8580", "ti,lp8863", "ti,lp8880"
  - reg: 0x2c for I2C slave address or SPI chip select address.
  - region-map: Region map code for LED 0 ~ LED5. Region map code is 0 to 5.
                In case of PWM mode, then only single region should be mapped.

Optional properties:
  - advanced-slope-enabled: Advanced slope effect.
  - enable-gpios: GPIO specifiers for HW enable pin.
  - irq-gpios: GPIO specifiers for fault interrupt pin.
  - pwms: PWM specifiers for backlight control.
  - slope-time-ms: Lighting effect. Slope time from 0 to 500.
  - names: Array of backlight driver name strings, in order, for each LED group.
  - max-input-brt: Maximum expected brightness value passed to the driver.
    This value is used to scale requested brightness values to the device's
    actual maximum brightness.
  - init-brt: Initial backlight brightness (applied to all LED groups). Must be
    scaled to max-input-brt, if provided, or otherwise max device brightness.

Example 1)
I2C for device configuration, map region 0 to all LEDs and PWM input for
backlight control.

	i2c1: i2c@f0018000 {
		backlight: lp8580@2c {
			compatible = "ti,lp8580";
			reg = <0x2c>;
			region-map = <0 0 0 0 0 0>;

			advanced-slope-enabled;
			enable-gpios = <&pioC 16 GPIO_ACTIVE_HIGH>;
			pwms = <&pwm0 0 10000 0>;
			slope-time-ms = <300>;
		};
	};

Example 2)
SPI for device configuration, CS is 0, backlight control through SPI,
interrupt handling, region 0 for LED 0, 1, 2, region 1 for LED 3, 4, 5.

	spi1: spi@f8008000 {
		cs-gpios = <&pioC 25 0>;
		status = "okay";

		backlight_spi: lp8880@0 {
			compatible = "ti,lp8880";
			spi-max-frequency = <8000000>;
			reg = <0>;

			region-map = <0 0 0 1 1 1>;
			slope-time-ms = <300>;
			advanced-slope-enabled;
			enable-gpios = <&pioC 16 GPIO_ACTIVE_HIGH>;
			irq-gpios = <&pioB 25 GPIO_ACTIVE_LOW>;
		};
	};