serial: imx: Add support for RS485 RX_DURING_TX output GPIO

If a RX_DURING_TX GPIO is defined by the DT property "rs485-rx-during-tx-gpios"
this patch switches this GPIO accordingly to the RS485 flag RX_DURING_TX in user
space. In addition, the i.MX UART receiver is no longer turned on and off during
sending, because now the hardware is responsible for connecting or disconnecting
RX during TX controlled by this GPIO.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Link: https://lore.kernel.org/r/20221202104127.122761-3-cniedermaier@dh-electronics.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>