serial: imx: implement DSR irq handling for DTE mode

Enable reporting of DSR events (which is named DTR in the registers
because Freescale uses the names as seem from a DCE).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>