arm: mvebu: turris_omnia: Enable networking via ethernet switch

The Turris Omnia contains the Marvell 88E6176 ethernet switch. Add
config options and device tree to enable the support.

Signed-off-by: Marek Mojík <marek.mojik@nic.cz>
Signed-off-by: Marek Behún <kabel@kernel.org>