leds: lm3692x: Introduce LM3692x dual string driver

Introducing the LM3692x Dual-String white LED driver.

Data sheet is located
http://www.ti.com/lit/ds/snvsa29/snvsa29.pdf



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