hw/net/xilinx_ethlite: Map MDIO registers (as unimplemented)

hw/net/xilinx_ethlite: Map MDIO registers (as unimplemented)

Rather than handling the MDIO registers as RAM, map them
as unimplemented I/O within the device MR.

The memory flat view becomes:

  (qemu) info mtree -f
  FlatView #0
   Root memory region: system
    0000000081000000-00000000810007e3 (prio 0, i/o): xlnx.xps-ethernetlite
    00000000810007e4-00000000810007f3 (prio 0, i/o): ethlite.mdio
    00000000810007f4-0000000081001fff (prio 0, i/o): xlnx.xps-ethernetlite @00000000000007f4

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Message-Id: <20241112181044.92193-7-philmd@linaro.org>