serial: imx: Support common rs485 binding for RTS polarity

Invoke the ->rs485_config callback on probe to adjust the initial RTS
polarity based on the UART's device properties.

This implicitly fixes a bug:  If RTS control is not available, rs485
should be disabled even if it was enabled through a device property.
Log an error when that occurs.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>