video: backlight: add Skyworks/Analogictech AAT2870 led controller driver

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>
3 files changed