KTD3137-backlight bindings

Required properties:
- compatible			: Must be "sprd,ktd3137"

Optional properties:
- hwen-gpio			: hwen gpio
- ktd,using-lsb			: switch max brightness
- ktd,using-linear		: switch update value
- ktd,bl-fscal-led		: default 20.2mA
- ktd,turn-on-ramp		: please check datasheet
- ktd,turn-off-ramp		: please check datasheet
- ktd,i2c-trans-dim		: please check datasheet
- ktd,bl-channel		: switch blacklight channel
- ktd,ovp-level			: default 32v
- ktd,inductor-current		: dfault 2600mA
- ktd,switching-frequency	: dfault 1000KHz

Example:

	ktd3137_backlight: sprd_backlight {
		status = "okay";
		compatible = "sprd,ktd3137";
		reg = <0x36>;
		hwen-gpio = <&ap_gpio 133 GPIO_ACTIVE_HIGH>;
		ktd,using-lsb;
		ktd,using-linear;
		ktd,bl-fscal-led = <0x99>;
		ktd,turn-on-ramp = <0>;
		ktd,turn-off-ramp = <0>;
		ktd,i2c-trans-dim = <0>;
		ktd,bl-channel = <2>;
		ktd,ovp-level = <32>;
		ktd,inductor-current = <1800>;
		ktd,switching-frequency = <1000>;
	};
