serial: imx: Allow passing 'rst-gpios' for rs485 mode

According to Documentation/devicetree/bindings/serial/serial.txt the
generic 'rts-gpios' property can be used to specify the GPIO for RTS
functionality.

Currently it is not possible to use the imx UART port in rs485 mode when
the 'rts-gpios' property is passed in the device tree.

The imx uart driver only checks for the presence of the built-in RTS pin,
via 'uart-has-rtscts' property and disable the rs485 flag if this property
is absent.

So fix this logic by also checking if RTS pin has been passed via GPIO.

Tested on a imx6dl based board.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -205,6 +205,7 @@ struct imx_port {
 	struct timer_list	timer;
 	unsigned int		old_status;
 	unsigned int		have_rtscts:1;
+	unsigned int		have_rtsgpio:1;
 	unsigned int		dte_mode:1;
 	unsigned int		irda_inv_rx:1;
 	unsigned int		irda_inv_tx:1;
@@ -1725,7 +1726,7 @@ static int imx_rs485_config(struct uart_port *port,
 	rs485conf->delay_rts_after_send = 0;
 
 	/* RTS is required to control the transmitter */
-	if (!sport->have_rtscts)
+	if (!sport->have_rtscts && !sport->have_rtsgpio)
 		rs485conf->flags &= ~SER_RS485_ENABLED;
 
 	if (rs485conf->flags & SER_RS485_ENABLED) {
@@ -2048,6 +2049,9 @@ static int serial_imx_probe_dt(struct imx_port *sport,
 	if (of_get_property(np, "fsl,dte-mode", NULL))
 		sport->dte_mode = 1;
 
+	if (of_get_property(np, "rts-gpios", NULL))
+		sport->have_rtsgpio = 1;
+
 	return 0;
 }
 #else