dt-bindings: arm: add entries for I2SE boards

This patch adds the missing devicetree binding documentation for I2SE's
Duckbill and Duckbill 2 series boards.
Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/i2se.txt

+I2SE Device Tree Bindings
+-------------------------
+
+Duckbill Board
+Required root node properties:
+    - compatible = "i2se,duckbill", "fsl,imx28";
+
+Duckbill 2 Board
+Required root node properties:
+    - compatible = "i2se,duckbill-2", "fsl,imx28";
+
+Duckbill 2 485 Board
+Required root node properties:
+    - compatible = "i2se,duckbill-2-485", "i2se,duckbill-2", "fsl,imx28";
+
+Duckbill 2 EnOcean Board
+Required root node properties:
+    - compatible = "i2se,duckbill-2-enocean", "i2se,duckbill-2", "fsl,imx28";
+
+Duckbill 2 SPI Board
+Required root node properties:
+    - compatible = "i2se,duckbill-2-spi", "i2se,duckbill-2", "fsl,imx28";