The hikey and hikey960 devboards let you build your own code to run in the trustzone - eg the boot chain isn't signed and secure. The trustzone and uefi implemention these boards use is also fully open source.
Maybe we should start with stating that the core of TZ is an execution mode of the ARM CPU privileged above the normal kernel mode, not a separate core running some obscure code. TZ can run any suitable hypervisor or just be left unused. It's nothing like the ME/PSP.
Still, some boards/devices may lock it down, for example by code signing.
I think it gets obfuscated due to the fact that about the only public information that AMD provides on the PSP is that it uses ARM Trustzone technology.
Still, some boards/devices may lock it down, for example by code signing.