leds: lm3532: Introduce the lm3532 LED driver

Introduce the Texas Instruments LM3532 White LED driver.
The driver supports ALS configurability or manual brightness
control.

The driver also supports associating LED strings with specific
control banks in a group or as individually controlled strings.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>