DT: Add documentation for the Skyworks AAT1290

* Skyworks Solutions, Inc. AAT1290 Current Regulator for Flash LEDs

The device is controlled through two pins: FL_EN and EN_SET. The pins when,

asserted high, enable flash strobe and movie mode (max 1/2 of flash current)

respectively.

Required properties:

- compatible : Must be "skyworks,aat1290".

- flen-gpios : Must be device tree identifier of the flash device FL_EN pin.

- enset-gpios : Must be device tree identifier of the flash device EN_SET pin.

A discrete LED element connected to the device must be represented by a child

node - see Documentation/devicetree/bindings/leds/common.txt.

Required properties of the LED child node:

- led-max-microamp : see Documentation/devicetree/bindings/leds/common.txt

- flash-max-microamp : see Documentation/devicetree/bindings/leds/common.txt

                       Maximum flash LED supply current can be calculated using

                       following formula: I = 1A * 162kohm / Rset.

- flash-timeout-us : see Documentation/devicetree/bindings/leds/common.txt

                     Maximum flash timeout can be calculated using following

                     formula: T = 8.82 * 10^9 * Ct.

Optional properties of the LED child node:

- label : see Documentation/devicetree/bindings/leds/common.txt

Example (by Ct = 220nF, Rset = 160kohm):

aat1290 {

	compatible = "skyworks,aat1290";

	flen-gpios = <&gpj1 1 GPIO_ACTIVE_HIGH>;

	enset-gpios = <&gpj1 2 GPIO_ACTIVE_HIGH>;

	camera_flash: flash-led {

		label = "aat1290-flash";

		led-max-microamp = <520833>;

		flash-max-microamp = <1012500>;

		flash-timeout-us = <1940000>;

	};

};