Xen on Pi4: Xen doesn't work with overlays from Raspberry Pi 5.4 kernel

[Corey Minyard <cminyard@xxxxxxxxxx>]

I had been working on Xen on the Pi4 by throwing kernels I compiled onto
existing sd cards, and this was working fine.  I finally got to a full
yocto build of the system, and it didn't boot.

In fact, Xen didn't print anything at all, and nothing happens that
might suggest it's booting without any console output.

I traced the issue down to the vc4-fkms-v3d dtoverly.  With everything
else the same, the 4.19 version of that overlay works, and the 5.4
version does not work.  It also didn't work if I completely removed the
overlay.  The base device trees are the same between the two kernels.

Looking at the overlay changes between the versions and Xen source, I
can't trace down anything that would cause an issue.  Has anyone seen
this issue of have any ideas?

-corey