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 <firstname.lastname@example.org>
Sponsored by: Intel Corporation
MFC after: 1 week