arm: mvebu: Espressobin: Use DM registered MDIO to configure switch

In order to be able to get rid of the non-DM MDIO bus registered in
mvneta driver, we need to stop using board_network_enable() and instead
use the DM registered MDIO device to configure switch in
last_stage_init().

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>