The wakeup mechanism via RTSDEN bit relies on the system using the RTS/CTS
lines, so only allow such wakeup method when the system actually has
RTS/CTS support.

Fixes: bc85734b ("serial: imx: allow waking up on RTSD")
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: NMartin Kaiser <martin@kaiser.cx>
Acked-by: NFugang Duan <fugang.duan@nxp.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>