libnvdimm/of_pmem: Provide a unique name for bus provider

libnvdimm/of_pmem: Provide a unique name for bus provider

ndctl binaries, v66 and older, mistakenly require the ndbus to have
unique names. If not while enumerating the bus in userspace it drops bus
with similar names.  This results in us not listing devices beneath the
bus.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Link: https://lore.kernel.org/r/20190807040029.11344-1-aneesh.kumar@linux.ibm.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>