boards: arm: nucleo: Add leds label

The nucleo evaluation boards can be enhanced with shields for additional
functionality. Flat device tree overlays can be used to configure and
support these shields. Regrettably leds can not be simply added due to
a missing label. Tag leds with a label.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Reviewed-by: Vasilij Strassheim <v.strassheim@linutronix.de>