libnvdimm, pmem, pfn: move pfn setup to the core

Now that pmem internals have been disentangled from pfn setup, that code
can move to the core.  This is in preparation for adding another user of
the pfn-device capabilities.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>