doc: efi-stub.txt: Fix arm64 paths

Update documented paths for arm64 files to match current tree.

Signed-off-by: Alan Ott <alan@softiron.co.uk>
Cc: Roy Franz <roy.franz@linaro.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>