tty/serial: atmel_serial: Fix device tree documentation

RTS pin is an active low pin.
For now, this doesn't change anything as the ACTIVE_LOW flag is not
handled in atmel_serial, but it will be in 3.16.
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/atmel-usart.txt

@@ -35,7 +35,7 @@ Example:
 		clock-names = "usart";
 		atmel,use-dma-rx;
 		atmel,use-dma-tx;
-		rts-gpios = <&pioD 15 0>;
+		rts-gpios = <&pioD 15 GPIO_ACTIVE_LOW>;
 	};
 
 - use DMA: