* Atmel Universal Synchronous Asynchronous Receiver/Transmitter (USART)

Required properties:
- compatible: Should be "atmel,<chip>-usart"
  The compatible <chip> indicated will be the first SoC to support an
  additional mode or an USART new feature.
- reg: Should contain registers location and length
- interrupts: Should contain interrupt
- clock-names: tuple listing input clock names.
	Required elements: "usart"
- clocks: phandles to input clocks.

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
- rts-gpios: specify a GPIO for RTS line. It will use specified PIO instead of the peripheral
  function pin for the USART RTS feature. If unsure, don't specify this property.
- 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>;
		interrupts = <7>;
		clocks = <&usart0_clk>;
		clock-names = "usart";
		atmel,use-dma-rx;
		atmel,use-dma-tx;
		rts-gpios = <&pioD 15 0>;
	};

- use DMA:
	usart0: serial@f001c000 {
		compatible = "atmel,at91sam9260-usart";
		reg = <0xf001c000 0x100>;
		interrupts = <12 4 5>;
		clocks = <&usart0_clk>;
		clock-names = "usart";
		atmel,use-dma-rx;
		atmel,use-dma-tx;
		dmas = <&dma0 2 0x3>,
		       <&dma0 2 0x204>;
		dma-names = "tx", "rx";
	};