serial: add device tree binding documentation for ETRAX FS UART

Signed-off-by: Niklas Cassel <nks@flawful.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/serial/axis,etraxfs-uart.txt

+ETRAX FS UART
+
+Required properties:
+- compatible : "axis,etraxfs-uart"
+- reg: offset and length of the register set for the device.
+- interrupts: device interrupt
+
+Optional properties:
+- {dtr,dsr,ri,cd}-gpios: specify a GPIO for DTR/DSR/RI/CD
+  line respectively.
+
+Example:
+
+serial@b00260000 {
+	compatible = "axis,etraxfs-uart";
+	reg = <0xb0026000 0x1000>;
+	interrupts = <68>;
+	status = "disabled";
+};

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -28,6 +28,7 @@ atmel	Atmel Corporation
 auo	AU Optronics Corporation
 avago	Avago Technologies
 avic	Shanghai AVIC Optoelectronics Co., Ltd.
+axis	Axis Communications AB
 bosch	Bosch Sensortec GmbH
 brcm	Broadcom Corporation
 buffalo	Buffalo, Inc.