arm64: replace pa_to_pvh() with page_to_pvh() in pmap_remove_l2()

Revise pmap_remove_l2() to use the constant-time function page_to_pvh()
instead of the linear-time function pa_to_pvh().

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30876