Add support for National Semiconductor/TI LP8550/1/2/3/5/6/7
LED Backlight. Driver is based on Linux version but is
reworked and optimised for U-Boot DM framework. Currently
only register driven backlight control is supported, PWM
driver backlight control may be added later if needed.
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
