arm64: imx8mp: Auto-detect PHY on i.MX8MP DHCOM

arm64: imx8mp: Auto-detect PHY on i.MX8MP DHCOM

The i.MX8MP DHCOM SoM may be populated with either KSZ9131RNXI RGMII PHY
or LAN8740Ai RMII PHY attached to EQoS MAC, and either external RGMII PHY
or LAN8740Ai RMII PHY attached to FEC MAC. The SoM configuration can be
detected for each MAC by reading RX_CTL pull resistor state early on boot.
Make use of this, detect the exact PHY configuration, and patch control DT
accordingly so that the ethernet is configured correctly in U-Boot.

Signed-off-by: Marek Vasut <marex@denx.de>