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

(cherry picked from commit 0c188c06c627b5de30eeeeb7cde00d071a80ecfa)