dt: bindings: lm3692x: Update binding for LED sync control

Update the binding to be able to configure the LED
sync to turn on.

The LM36922 has one output but can sync current to 2 LED
strings. The user may only use one sync so the other
syncs need to be disabled.

The LM36923 has 3 LED syncs.

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