Try to see if it is software issue or hardware issue is to configure the loopback mode by setting GUMR[DIAG] = 01. If the lookback works, then issue is with hardware: pin configuration error, clock, PHY, or signal problem; if the loopback fails, it is with software configuration problem.
Check also Device Tree Structure of your BSP.
See the following pages:
https://www.nxp.com/docs/en/application-note/AN5125.pdf
https://www.kernel.org/doc/Documentation/devicetree/usage-model.txt
https://stackoverflow.com/questions/21670967/how-to-compile-device-tree-source-files-dts
Have a great day,
Pavel Chubakov
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