MFC 348757,351225: acpi and nvdimm location_str updates

348757:
nvdimm: Provide nvdimm location information

Provide the acpi handle path as the location string for the nvdimm
children of the nvdimm_root device.

351225:
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.

Approved by: scottl (mentor)
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D20644