arm64: Make a debug print conditional on bootverbose

It doesn't seem particularly useful to have it enabled by default, and
it spits several dozen lines into the dmesg on a test system.

Reviewed by: andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40499

(cherry picked from commit 50e1cc94a138f8886588eabe0b849b487d9925a3)