Add support for Skyworks AAT2870 LED Backlight Driver and Multiple LDO
Lighting Management Unit. Only backlight is supported as for now. Supported
backlight level range is from 2 to 255 with step of 1.
Tested-by: Ion Agorria <ion@agorria.com>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>