acpi/nfit: Add sysfs "id" for NVDIMM ID

ACPI 6.1, section 5.2.25.9, defines an identifier for an NVDIMM.

Change the NFIT driver to add a new sysfs file "id" under nfit
directory.

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Robert Moore <robert.moore@intel.com>
Cc: Robert Elliott <elliott@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>