UPDATE: I compiled the MiSTer Linux kernel, which integrates ALSA USB audio drivers from here:
I have two simple USB audio interfaces working now! From the kernel log output of the MiSTer kernel, I see one additional line, that was not in the standard linux-socfpa kernel (as far as I remember):
“dwc2 ffb40000.usb: Linked as a consumer to regulator.0”
Could that be part of the solution that the MiSTer developers worked out?