serial/imx: make imx_port.devdata member point to const data

This prepares *of_device_id.data becoming const. Without this change the
following warning would occur:

	drivers/tty/serial/imx.c: In function 'serial_imx_probe_dt':
	drivers/tty/serial/imx.c:1430:17: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/tty/serial/imx.c

@@ -206,7 +206,7 @@ struct imx_port {
 	unsigned short		trcv_delay; /* transceiver delay */
 	struct clk		*clk_ipg;
 	struct clk		*clk_per;
-	struct imx_uart_data	*devdata;
+	const struct imx_uart_data *devdata;
 };
 
 struct imx_port_ucrs {