There’s a new mirror of the Google Fuschsia UEFI headers:
https://github.com/no92/zircon-uefi
See-also:
https://fuchsia.googlesource.com/zircon/+/master/docs/ddk/driver-development.md
https://en.wikipedia.org/wiki/Google_Fuchsia
https://firmwaresecurity.com/2016/08/15/uefi-bootloader-for-google-fuchsia/