Fix invalid root link detection in mv_pci driver

mv_pci driver omitted slot 0, which can be valid device on Armada38x.
New mechanism detects if device is root link, basing on vendor's
and device's IDs.
It is restricted to Armada38x; on other machines, behaviour remains
the same.

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4377