leds: lp8860: Introduce TI lp8860 4 channel LED driver

Introduce the Texas Instruments lp8860
4 channel LED driver.

This driver configures the device in display cluster mode
as this seems to be the most used configuration at the
time of the driver configuration.

For more product information please see the link below:
http://www.ti.com/product/lp8860-q1

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>