libnvdimm: Update persistence domain value for of_pmem and papr_scm device

Currently, kernel shows the below values
	"persistence_domain":"cpu_cache"
	"persistence_domain":"memory_controller"
	"persistence_domain":"unknown"

"cpu_cache" indicates no extra instructions is needed to ensure the persistence
of data in the pmem media on power failure.

"memory_controller" indicates cpu cache flush instructions are required to flush
the data. Platform provides mechanisms to automatically flush outstanding
write data from memory controler to pmem on system power loss.

Based on the above use memory_controller for non volatile regions on ppc64.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/20200324034821.60869-1-aneesh.kumar@linux.ibm.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>