libnvdimm, pfn: enable pfn sysfs interface unit testing

libnvdimm, pfn: enable pfn sysfs interface unit testing

Orabug: 22913653

The unit test infrastructure uses CMA and real memory to emulate nvdimm
resources.  The call to devm_memremap_pages() can simply be mocked in
the same manner as memremap and we mock phys_to_pfn_t() to clear PFN_MAP
since these resources are not registered with in the pgmap_radix.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
(cherry picked from commit 979fccfb7348dbd968daf0249aa484a0297f83df)
Signed-off-by: Dan Duval <dan.duval@oracle.com>