LS1028a has one Ethernet management interface. On the QDS board, the
MDIO signals are multiplexed to either on-board AR8035 PHY device or
to 4 PCIe slots allowing for SGMII cards.
To enable the Ethernet ENETC Port 1, which can only be connected to a
RGMII PHY, the multiplexer needs to be configured to route the MDIO to
the AR8035 PHY.  The MDIO/MDC routing is controlled by bits 7:4 of FPGA
board config register 0x54, and value 0 selects the on-board RGMII PHY.
The FPGA board config registers are accessible on the i2c bus, at address
0x66.

The PF3 MDIO PCIe integrated endpoint device allows for centralized access
to the MDIO bus.  Add the corresponding devicetree node and set it to be
the MDIO bus parent.
Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>