nvdimm: Provide nvdimm location information
Provide the acpi handle path as the location string for the nvdimm
children of the nvdimm_root device.
Don't set the string "unknown" as a device's location_str
Return an empty string when the location is unknown instead of the
string "unknown". This ensures that all location entries are of
the form key=val.
Sponsored by: Intel Corporation