nvdimm: split spa dev into a separate entity

Separate code for exposing a device backed by a system physical
address range away from the NVDIMM spa code. This will allow a
future patch to add support for NVDIMM namespaces while using the
same device code.

Submitted by: D Scott Phillips <d.scott.phillips@intel.com>
Reviewed by: bwidawsk
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D18736