libnvdimm, dax: introduce device-dax infrastructure

libnvdimm, dax: introduce device-dax infrastructure

Orabug: 22913653

Device DAX is the device-centric analogue of Filesystem DAX
(CONFIG_FS_DAX).  It allows persistent memory ranges to be allocated and
mapped without need of an intervening file system.  This initial
infrastructure arranges for a libnvdimm pfn-device to be represented as
a different device-type so that it can be attached to a driver other
than the pmem driver.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dan Duval <dan.duval@oracle.com>
(cherry picked from commit cd03412a51ac4cb3001a8cdfae4560c9602f3387)

Conflicts:

	drivers/nvdimm/namespace_devs.c
	drivers/nvdimm/pfn_devs.c