serial: imx: allow waking up on RTSD

This patch sets RTSDEN bit when going into idle (Stop mode).
We add the RTSDEN for the case RTS is sent from
the remote connection. This way we allow the system
to wakeup when RTS is received.

Cc: Fabio Stevam <festevam@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>