acpi, nfit: allow specifying a default DSM family

Provide the ability to request a default DSM family. If it is not
supported, then fall back to the normal discovery order.

This is helpful for testing platforms that support multiple DSM
families.  It will also allow administrators to request the DSM family
that their management tools support, which may not be the first one
found using the current discovery order.

Signed-off-by: Linda Knippers <linda.knippers@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>