Time to test it.
After compilation of image:
here we see that both ethernet controllers 20b4000 and 2188000 initialized.
Let’s try to configure networks.
Step 5. Network configuration.
Seems that DTS used by u-boot unable to initialize properly at least first PHY IC in a case of MDIO shared between two ICs. Again need to modify DTS but for u-boot now.
Step 6. U-boot’s DTS modification.
Looks similar to Linux’s DTS but PHY IC address on MDIO bus is 2. Seems it is a mistake. Changed it to 0, recompiled image again, updated firmware and checked PHY status again: