libnvdimm, dax: introduce device-dax infrastructure

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>
diff --git a/drivers/nvdimm/Makefile b/drivers/nvdimm/Makefile
index ea84d3c..909554c 100644
--- a/drivers/nvdimm/Makefile
+++ b/drivers/nvdimm/Makefile
@@ -23,3 +23,4 @@
 libnvdimm-$(CONFIG_ND_CLAIM) += claim.o
 libnvdimm-$(CONFIG_BTT) += btt_devs.o
 libnvdimm-$(CONFIG_NVDIMM_PFN) += pfn_devs.o
+libnvdimm-$(CONFIG_NVDIMM_DAX) += dax_devs.o