leds: leds-gpio: Fix multiple instances registration without 'label' property

Since commit a43f2cbb ("leds: leds-gpio: Make use of device property
API") it is no longer possible to register multiple gpio leds without passing
the 'label' property.

According to Documentation/devicetree/bindings/leds/common.txt:

"Optional properties for child nodes:
- label : The label for this LED.  If omitted, the label is
  taken from the node name (excluding the unit address)."

So retrieve the node name when the 'label' property is absent to keep the old
behaviour and fix this regression.

Fixes: a43f2cbb (leds: leds-gpio: Make use of device property API)
Reported-by: Jean-Michel Hautbois <jean-michel.hautbois@vodalys.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>