serial: imx: Use common rs485 device tree parsing function

This adds support for the rs485 specific properties defined in
Documentation/devicetree/bindings/serial/rs485.txt.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Forwarded: id:20170913081833.2740-7-u.kleine-koenig@pengutronix.de (v5)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>