arm64: make VM_NFREEORDER and the comment describing it match

The setting of VM_NFREEORDER and the comment describing it were copied
from sparc64 where both the page size and the number of page table
entries that fit in a cache line are different from arm64.

Reviewed by: andrew, kib, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D40782