Add GEOM attribute to report physical device name, and report it
via 'diskinfo -v'. This avoids the need to track it down via CAM,
and should also work for disks that don't use CAM. And since it's
inherited thru the GEOM hierarchy, in most cases one doesn't need
to walk the GEOM graph either, eg you can use it on a partition
instead of disk itself.

Reviewed by: allanjude, imp
Sponsored by: Klara Inc
Differential Revision: https://reviews.freebsd.org/D22249