hw/net/xilinx_ethlite: Map RX_CTRL as MMIO

hw/net/xilinx_ethlite: Map RX_CTRL as MMIO

Declare RX registers as MMIO region, split it out
of the current mixed RAM/MMIO region.
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
    00000000810007f4-00000000810017fb (prio 0, i/o): xlnx.xps-ethernetlite @00000000000007f4
    00000000810017fc-00000000810017ff (prio 0, i/o): ethlite.rx[0]io
    0000000081001800-0000000081001ffb (prio 0, i/o): xlnx.xps-ethernetlite @0000000000001800
    0000000081001ffc-0000000081001fff (prio 0, i/o): ethlite.rx[1]io

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20241112181044.92193-16-philmd@linaro.org>