leds: lm3697: Introduce the lm3697 driver

Introduce the lm3697 LED driver for backlighting and display.

Datasheet location:
http://www.ti.com/lit/ds/symlink/lm3697.pdf



Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>