what are the RGMII interface voltage levels of the i.mx8 Apalis Modul pins ? 2,5V or 3,3V?
Those signals are 3.3V.
Can the RGMII signals be referenced to 2.5V?
with the IMX8QM NXP changed the specification of the interface. If you want to use the RGMMI interface on the IMX8QM you have to use 1.8 or 2.5 volts. this can be switched in the device tree.
If you just want to use RMII that you can switch the interface to 3.3 volts.
But keep in mind if you use 2.5 or 1.8 volt mode the MDIO signal of the RGMII port will be still 3.3 volt and you need to level shift it.
If you need some ref. schematic on this please write an email to email@example.com and request it.