ARM: dts: turris-omnia: add support for ethernet switch

The Turris Omnia features a Marvell MV88E6176 ethernet switch. Add it to
the dts.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>