firmware: dmi_scan: Add DMI_OEM_STRING support to dmi_matches

OEM strings are defined by each OEM and they contain customized and
useful OEM information. Supporting it provides more flexible uses of
the dmi_matches function.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>