serial: at91: add dma support in usart binding descriptions

Signed-off-by: NElen Song <elen.song@atmel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -10,13 +10,18 @@ Required properties:
 Optional properties:
 - atmel,use-dma-rx: use of PDC or DMA for receiving data
 - atmel,use-dma-tx: use of PDC or DMA for transmitting data
+- add dma bindings for dma transfer:
+	- dmas: DMA specifier, consisting of a phandle to DMA controller node,
+		memory peripheral interface and USART DMA channel ID, FIFO configuration.
+		Refer to dma.txt and atmel-dma.txt for details.
+	- dma-names: "rx" for RX channel, "tx" for TX channel.
 
 <chip> compatible description:
 - at91rm9200:  legacy USART support
 - at91sam9260: generic USART implementation for SAM9 SoCs
 
 Example:
-
+- use PDC:
 	usart0: serial@fff8c000 {
 		compatible = "atmel,at91sam9260-usart";
 		reg = <0xfff8c000 0x4000>;
@@ -25,3 +30,14 @@ Example:
 		atmel,use-dma-tx;
 	};
 
+- use DMA:
+	usart0: serial@f001c000 {
+		compatible = "atmel,at91sam9260-usart";
+		reg = <0xf001c000 0x100>;
+		interrupts = <12 4 5>;
+		atmel,use-dma-rx;
+		atmel,use-dma-tx;
+		dmas = <&dma0 2 0x3>,
+		       <&dma0 2 0x204>;
+		dma-names = "tx", "rx";
+	};