dt: bindings: lm3692x: Add bindings for lm3692x LED driver

This adds the devicetree bindings for the LM3692x
I2C LED string driver.

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>