ARM: Kirkwood: add LED DT entries for netxbig boards

This patch adds DT entries for the LEDs found on the Kirkwood-based
LaCie boards 2Big and 5Big Network v2.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>