net: xilinx: Use mdio_register_seq() to support multiple instances

axi_emac, emaclite and gem have the same issue with registering
multiple instances with mdio busses. mdio bus name has to be uniq but
drivers are setting up only one name for all.
Use mdio_register_seq() and pass dev->seq number to allow multiple
mdio instances registration.

Reported-by: Phani Kiran Kara <phanikiran.kara@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Series-to: u-boot
Series-cc: Phani Kiran Kara <phanikiran.kara@gmail.com>