ARM: dts: at91: add devicetree for the Axentia Nattis with Natte power

The Axentia Nattis is a device designed for presenting departures for
public transport systems. The Natte helper board provides power and
features a battery of battery chargers.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>