libnvdimm, pmem: fix ia64 build, use PHYS_PFN

   drivers/nvdimm/pmem.c: In function 'nvdimm_namespace_attach_pfn':
   drivers/nvdimm/pmem.c:367:3: error: implicit declaration of function
   	'__phys_to_pfn' [-Werror=implicit-function-declaration]
   .base_pfn = __phys_to_pfn(nsio->res.start),

ia64 does not provide __phys_to_pfn(), just use the PHYS_PFN() alias.

Cc: Guenter Roeck <linux@roeck-us.net>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>