serial: at91: add dma support in usart binding descriptions

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>;

		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";

	};