I don't believe there are any POWER machines using UEFI. The standard POWER boot process uses OPAL (Open Power Abstraction Layer) instead. But POWER does use GPT disk partitioning, which is from the UEFI standard.
Not a lot of advantages in UEFI over petitboot, though.
Edit: here it is. https://github.com/andreiw/ppc64le-edk2 mocks the presence of ACPI tables using the equivalent OPAL calls.