arm64/machdep: Add parameter to the EFI table walking code

It would be nice to be able to pass an arbitrary pointer to the callback
code. Add one, and pass NULL in all the places that we do that today.
As noted by andrew@, we should likely refactor this into MI code and use
it here and amd64, but for the future.

Sponsored by: Netflix
Reviewed by: rpokala
Differential Revision: https://reviews.freebsd.org/D37439

(cherry picked from commit 6849950da7d770fa6531b915922799c20e960d42)