Add initial driver for ACPI NFIT-enumerated NVDIMMs.

Driver enumerates NVDIMMs. Besides, for each found System Physical
Address (SPA) range, spaN geom provider is created, which allows
formatting and mounting the region as the normal volume. Also,
/dev/nvdimm_spaN node is created, which can be read/written/mapped by
userspace, the mapping is zero-copy.

No support for block access methods implemented, labels are not
parsed. No management interfaces are provided.

Tested by: Intel, NetApp
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
MFC after: 2 weeks