serial: 8250_pxa: honor the port number from devicetree

Like the other OF-enabled drivers, use the port number from the firmware if
the devicetree specifies an alias:

  aliases {
      ...
      serial2 = &uart2; /* Should be ttyS2 */
  }

This is how the deprecated pxa.c driver behaved, switching to 8250_pxa
messes up the numbering.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_pxa.c

@@ -113,6 +113,10 @@ static int serial_pxa_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	ret = of_alias_get_id(pdev->dev.of_node, "serial");
+	if (ret >= 0)
+		uart.port.line = ret;
+
 	uart.port.type = PORT_XSCALE;
 	uart.port.iotype = UPIO_MEM32;
 	uart.port.mapbase = mmres->start;