Add support for unspecified ranges on ThunderX system

When one tries to allocate a resource with unspecified range,
read already configured BAR values (by UEFI or whatever).
This is necessary to make VNIC VFs working and to allow them to be
properly allocated.

Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D3752