ARM: dts: add support for Turris Omnia

This machine is an open hardware router by cz.nic driven by a
Marvell Armada 385.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Tomas Hlavacek <tmshlvck@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>