firmware: dmi: Add access to the SKU ID string

This is used in some systems from user space for determining the identity
of the device.

Expose this as a file so that that user-space tools don't need to read
from /sys/firmware/dmi/tables/DMI

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jean Delvare <jdelvare@suse.de>