add fwupdate-serio user and group use by the ELAN TrackPoint firmware updater

BUG=b:162454660
TEST=none

Change-Id: Ifdaa6a204fa5e20d1c634ddee6293e76cd01f56d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2391895
Reviewed-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-by: Allen Webb <allenwebb@google.com>
Tested-by: jingle wu <jingle.wu@elan.corp-partner.google.com>
Commit-Queue: Raul E Rangel <rrangel@chromium.org>