Beside the "well-known" implementations of UEFI for x86-32, IA-64 (Itanium) and X64-64, there also exist official UEFI implementations for ARM AArch32 and ARM AArch64. According to https://en.wikipedia.org/w/index.php?title=Unified_Extensibl... there also exist inofficial implementations for POWERPC64, MIPS and RISC-V.
One consumer example where UEFI on ARM processors was used was the ARM-based Windows RT laptops (they were not very successful in the market). Much more importantly an UEFI implementation is required for ARM's "Server Base Boot Requirements (SBBR) - System Software on ARM® Platforms" standard:
> http://infocenter.arm.com/help/topic/com.arm.doc.den0044b/DE...
(read section 3 and appendices A to D). So about every ARM server uses an UEFI firmware.
The kind of systems using FDTs aren't going to switch I don't think. I have to imagine that ARM64 UEFI was done for marketing and by clueless product management. The CPUs and integration of ARM servers have been so pathetic, nobody was buying, and they were fumbling around at this level while ignoring the elephant in the room. TX2 and Centriq are the first realistic implementations for general purpose servers, and now they are unfortunately saddled with UEFI and ACPI.
This was a rare 2-3 decades kind of mistake. Not a lot of software gets that privilege.