Only know I found https://trustedfirmware-a.readthedocs.io/en/latest/getting_started/prerequisites.html.

https://developer.trustedfirmware.org/T984 suggests it's a problem with GCC, which was supposed to be fixed in GCC 12.1, but my GCC version is 12.2.0, and I can also reproduce those array subscript 0 is outside array bounds build errors in include/lib/mmio.h using the latest Arm GNU Toolchain from https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads, which is based on GCC 12.2:

$ aarch64-none-elf-gcc --version
aarch64-none-elf-gcc (Arm GNU Toolchain 12.2.Rel1 (Build arm-12.24)) 12.2.1 2022120