libnvdimm: Move region attribute group definition

libnvdimm: Move region attribute group definition

In preparation for moving region attributes from device attribute groups
to the region device-type, reorder the declaration so that it can be
referenced by the device-type definition without forward declarations.
No functional changes are intended to result from this change.

Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/157309900624.1582359.6929998072035982264.stgit@dwillia2-desk3.amr.corp.intel.com