Move the enumeration of NVDIMM SPA ranges from the spa GEOM class
initializer into the NVDIMM root device. This will be necessary
for a later change where NVDIMM namespaces require NVDIMM device
enumeration to be reliably ordered before SPA enumeration.
Submitted by:	D Scott Phillips <d.scott.phillips@intel.com>
Sponsored by:	Intel Corporation
MFC after:	1 week