serial: mxs-auart: add the DMA support for mx28

Only we meet the following conditions, we can enable the DMA support for
auart:

  (1) We enable the DMA support in the dts file, such as
      arch/arm/boot/dts/imx28.dtsi.

  (2) We enable the hardware flow control.

  (3) We use the mx28, not the mx23. Due to hardware bug(see errata: 2836),
      we can not add the DMA support to mx23.
Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/tty/serial/fsl-mxs-auart.txt

@@ -6,11 +6,19 @@ Required properties:
 - reg : Address and length of the register set for the device
 - interrupts : Should contain the auart interrupt numbers
 
+Optional properties:
+- fsl,auart-dma-channel : The DMA channels, the first is for RX, the other
+		is for TX. If you add this property, it also means that you
+		will enable the DMA support for the auart.
+		Note: due to the hardware bug in imx23(see errata : 2836),
+		only the imx28 can enable the DMA support for the auart.
+
 Example:
 auart0: serial@8006a000 {
 	compatible = "fsl,imx28-auart", "fsl,imx23-auart";
 	reg = <0x8006a000 0x2000>;
 	interrupts = <112 70 71>;
+	fsl,auart-dma-channel = <8 9>;
 };
 
 Note: Each auart port should have an alias correctly numbered in "aliases"