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>

Documentation/devicetree/bindings/leds/leds-lm3692x.txt

@@ -20,7 +20,10 @@ Optional properties:
 	- vled-supply : LED supply
 
 Required child properties:
-	- reg : 0
+	- reg : 0 - Will enable all LED sync paths
+		1 - Will enable the LED1 sync
+		2 - Will enable the LED2 sync
+		3 - Will enable the LED3 sync (LM36923 only)
 
 Optional child properties:
 	- label : see Documentation/devicetree/bindings/leds/common.txt