tty/serial: RS485 bindings for device tree

Generic bindings for RS485 feature included in some UARTs.
Those bindings have to be used withing an UART device tree node.
Documentation updated to link to the bindings definition.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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

@@ -0,0 +1,27 @@
+* 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
+
+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
+
+<chip> compatible description:
+- at91rm9200:  legacy USART support
+- at91sam9260: generic USART implementation for SAM9 SoCs
+
+Example:
+
+	usart0: serial@fff8c000 {
+		compatible = "atmel,at91sam9260-usart";
+		reg = <0xfff8c000 0x4000>;
+		interrupts = <7>;
+		atmel,use-dma-rx;
+		atmel,use-dma-tx;
+	};
+