Please check that you are able to access the RGMII PHY's registers in U-Boot.
In the described case it is convenient to create a Technical Case:
1) open www.nxp.com
2) Select "Support" -> "All Support Options"
3) Click "Go to Tickets"
4) Log in with your NXP login and password
5) On the "Group, Create and View your support cases" page press "+ Add a new case" to start the process.