nvdimm: add a driver for the NVDIMM root device

The NVDIMM root device is parent to the individual ACPI NVDIMM
devices. Add a driver for the NVDIMM root device that can own
enumeration of NVDIMM devices as well as NVDIMM SPA ranges that the
system has.

Submitted by: D Scott Phillips <d.scott.phillips@intel.com>
Sponsored by: Intel Corporation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18346