DT: Add documentation for the Skyworks AAT1290

This patch adds device tree binding documentation for
1.5A Step-Up Current Regulator for Flash LEDs.
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: devicetree@vger.kernel.org
Signed-off-by: Bryan Wu <cooloney@gmail.com>

Documentation/devicetree/bindings/leds/leds-aat1290.txt

+* 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>;
+	};
+};