net: zynq-gem: Setup and use mdio base separately

Not all IPs have private MDIO bus and MDIO bus should be shared between
several IPs. In past one patch tried to implement it
(https://lists.denx.de/pipermail/u-boot/2018-February/319285.html

)
in pretty raw way but it is not the cleanest solution.
This patch is just taking the part of that solution to be able to handle it
over releases without conflicts.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>