DT: leds: Add uniqueness requirement for 'label' property.

Label is used for naming LED class devices. Since ePAPR
doesn't require uniqueness for label properties, it has to be
explicitly required in the LEDs common bindings documentation.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: devicetree@vger.kernel.org
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

Documentation/devicetree/bindings/leds/common.txt

@@ -14,8 +14,10 @@ Optional properties for child nodes:
 - led-sources : List of device current outputs the LED is connected to. The
 		outputs are identified by the numbers that must be defined
 		in the LED device binding documentation.
-- label : The label for this LED.  If omitted, the label is
-  taken from the node name (excluding the unit address).
+- label : The label for this LED. If omitted, the label is taken from the node
+	  name (excluding the unit address). It has to uniquely identify
+	  a device, i.e. no other LED class device can be assigned the same
+	  label.
 
 - linux,default-trigger :  This parameter, if present, is a
     string defining the trigger assigned to the LED.  Current triggers are: