The "register" device is only used by certain machines. Let's add
a proper config switch for it so that it only gets compiled when we
really need it.

Message-Id: <20190817101931.28386-5-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>