Documentation: devicetree: Adapteva boards

We're about to add a device tree for the Parallella board.

Cc: Andreas Olofsson <andreas@adapteva.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/arm/adapteva.txt

+Adapteva Platforms Device Tree Bindings
+---------------------------------------
+
+Parallella board
+
+Required root node properties:
+    - compatible = "adapteva,parallella";