arm64: Refine comment about size of struct pmap_large_md_page

pmap_init_pv_table does not assume a specific size of 64. It does
assume a size that evenly divides the size of a page since it inserts
pages from different domains into the kva backing the global pv_table
array.

Reviewed by: imp, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D48455