leds: Add driver for AAT1290 flash LED controller

This patch adds a driver for the 1.5A Step-Up Current Regulator
for Flash LEDs. The device is programmed through a Skyworks proprietary
AS2Cwire serial digital interface.

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>
Signed-off-by: Bryan Wu <cooloney@gmail.com>