platform/x86: dell-laptop: Add quirk entry for Latitude 7520

The Latitude 7520 supports AC timeouts, but it has no KBD_LED_AC_TOKEN
and so changes to stop_timeout appear to have no effect if the laptop
is plugged in.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Acked-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20220426120827.12363-1-gabriele.mzt@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>