Thanks, I submitted the following based on CJKay's patch above:
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/9990

It's native aarch64, the command line I'm using is:
make HOSTCC="gcc $RPM_OPT_FLAGS" CROSS_COMPILE="" PLAT=rk3399 bl31

I'm using Fedora 34/35 with the distribution toolchain (gcc 11.1 GA)