xilinx: Fix mdio bus description for vck190-sc

Current behavior is that eth_phy_get_mdio_bus

Net:   FEC: can't find phy-handle

ZYNQ GEM: ff0b0000, mdio bus ff0b0000, phyaddr 0, interface sgmii
eth0: ethernet@ff0b0000

Net:
ZYNQ GEM: ff0b0000, mdio bus ff0b0000, phyaddr 0, interface sgmii
eth0: ethernet@ff0b0000

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/5b7da5808136b3f579c0cf7a3431b56c758655e9.1662460749.git.michal.simek@amd.com