serial: mxc: Add iMX6SX compatible string

Add compatible string for iMX6SX.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Silvio Fricke <silvio.fricke@softing.com>
Cc: Stefano Babic <sbabic@denx.de>

drivers/serial/serial_mxc.c

@@ -342,6 +342,7 @@ static int mxc_serial_ofdata_to_platdata(struct udevice *dev)
 }
 
 static const struct udevice_id mxc_serial_ids[] = {
+	{ .compatible = "fsl,imx6sx-uart" },
 	{ .compatible = "fsl,imx6ul-uart" },
 	{ .compatible = "fsl,imx7d-uart" },
 	{ .compatible = "fsl,imx6q-uart" },