ARM: highbank: add devicetree source

This adds the devicetree source and documentation for the Calxeda highbank
platform.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>

Documentation/devicetree/bindings/arm/calxeda.txt

@@ -0,0 +1,8 @@
+Calxeda Highbank Platforms Device Tree Bindings
+-----------------------------------------------
+
+Boards with Calxeda Cortex-A9 based Highbank SOC shall have the following
+properties.
+
+Required root node properties:
+    - compatible = "calxeda,highbank";