Input: i8042 - add Lenovo XiaoXin Air 12 to i8042 nomux list

This fixes two finger trackpad scroll on the Lenovo XiaoXin Air 12.
Without nomux, the trackpad behaves as if only one finger is present and
moves the cursor when trying to scroll.

Signed-off-by: David Pedersen <limero1337@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200625133754.291325-1-limero1337@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>