Support EFIRT on arm64 with a non 4k page size

When the page size the kernel is built for is not the same as
EFI_PAGE_SIZE we need to increment the page index at a faster rate.

Add this adjustment to the arm64 EFIRT support in preperation for
experimental 16k PAGE_SIZE support.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 863f3220d09716ea2e570fadc6b19dcb6e4c2191)