This issue is fixed in upstream u-boot by commit a246e21 ("arm64: dts: meson: odroidc2: readd PHY reset properties"), which is included in u-boot v2021.07-rc1. The regression was caused by the backport of a kernel commit that had change a reset timing too aggressively, and first went into u-boot v2020.07. See upstream discussion, openSUSE bug 1187095.