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>