serial: stm32: 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. Controlled by this GPIO, now the hardware is responsible for connecting
or disconnecting RX during TX.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Link: https://lore.kernel.org/r/20221202104127.122761-4-cniedermaier@dh-electronics.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -226,7 +226,11 @@ static int stm32_usart_config_rs485(struct uart_port *port, struct ktermios *ter
 
 	stm32_usart_clr_bits(port, ofs->cr1, BIT(cfg->uart_enable_bit));
 
-	rs485conf->flags |= SER_RS485_RX_DURING_TX;
+	if (port->rs485_rx_during_tx_gpio)
+		gpiod_set_value_cansleep(port->rs485_rx_during_tx_gpio,
+					 !!(rs485conf->flags & SER_RS485_RX_DURING_TX));
+	else
+		rs485conf->flags |= SER_RS485_RX_DURING_TX;
 
 	if (rs485conf->flags & SER_RS485_ENABLED) {
 		cr1 = readl_relaxed(port->membase + ofs->cr1);