I think that this issue is related to the fact that the Pi foundation has changed the default SD card driver to ‘SDHost’. It would be good to know which SD cards people are using, as there are still some compatibility issues.
I’d be interested in knowing if this resolves the issue:
Download this file: https://github.com/raspberrypi/firmware/raw/master/boot/overlays/mmc-overlay.dtb and place it in /boot/overlays. This can be done from a PC or Mac.
Edit config.txt, and add the following:
dtoverlay=mmc
Sam