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>