nfit: export subsystem ids as attributes

Similar to pci-sysfs export the subsystem information available in the
NFIT.  ACPI 6.1 clarifies that this data is copied as an array of bytes
from the DIMM SPD data.

Reported-by: Ryon Jensen <ryon.jensen@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>