drivers: ethernet: xlnx_gem: switch over to new MDIO driver

Remove all data/functions/function calls relating to the now
removed legacy MDIO/PHY support directly integrated into the
GEM device driver. Switch over to using the standard MDIO/PHY
interfaces which will now refer to the new separate MDIO
driver and the separated PHY drivers.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>

Author: ibirnbaum