Not sure how much interest there is in this (I'm sure folks would prefer to see a fully support Raspberry Pi 4 BSP!) but
 I did manage to get the Beaglebone (Black) running 7.1.  I can't attest to whether my changes to the audio driver 
actually work (not using it), but they do compile.  Only other change I needed to do was to include the smmu lib to one 
of the startup module Makefiles.  But seems to work fine.