TI LM3697 backlight device tree bindings

Required properties:
  - compatible: "ti,lm3697"

Optional properties: Only specified in PWM brightness control mode
  - pwms: OF device-tree PWM specification.
  - pwm-names: a list of names for the PWM devices specified in the "pwms"
               property.
  For the PWM user nodes, please refer to
  Documentation/devicetree/bindings/pwm/pwm.txt.

Sub-nodes:
  LM3697 backlight should have backlight device specifiers in each sub-node.
  Each sub-node exactly matches with backlight control bank configuration.
  Maximum number of sub-nodes is 2.

  Required properties of sub-node:
  - hvled1-used, hvled2-used, hvled3-used: Backlight string configuration about
                                           which backlight string is used.

  Optional properties of sub-node:
  - backlight-name: Name string for backlight device identification
  - max-current-milliamp: Max current setting. Type is <u32>. Unit is mA.
                          Range is from 5 to 30mA.
  - initial-brightness: Backlight initial brightness. Type is <u32>.
                        Range is from 0 to 2047.
  - pwm-period: PWM period. Only valid in PWM brightness mode. Type is <u32>.

Examples:

One backlight device is defined. It's PWM based control with period 10KHz.

lm3697@36 {
	compatible = "ti,lm3697";
	reg = <0x36>;

	/* GPIO134 for HWEN pin */
	ti,enable-gpio = <&gpio5 6 0>;

	pwms = <&pwm3943 1 10000>;
	pwm-names = "lm3697-backlight";

	lcd_bl {
		backlight-name = "lcd";
		hvled1-used;
		hvled2-used;
		hvled3-used;

		max-current-milliamp = <20>;
		initial-brightness = <2000>;
		pwm-period = <10000>;
	};
};

Brightness register base, two backlight devices are configured,
'lcd' and 'status'. The 'lcd' is connected to HVLED1 and 2.
The 'status' is connected to HVLED3.

lm3697@36 {
	compatible = "ti,lm3697";
	reg = <0x36>;

	/* GPIO134 for HWEN pin */
	ti,enable-gpio = <&gpio5 6 0>;

	lcd_bl {
		backlight-name = "lcd";
		hvled1-used;
		hvled2-used;

		max-current-milliamp = <20>;
		initial-brightness = <10>;
	};

	status {
		backlight-name = "status";
		hvled3-used;

		max-current-milliamp = <10>;
		initial-brightness = <20>;
	};
};
