nvdimm: enumerate NVDIMM SPA ranges from the root device

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
Differential revision: https://reviews.freebsd.org/D18734