Documentation/DT: add blurb for IB2 syscon to Versatile

The ARM Versatile has an optional daughterboard, if this is
mounted, we need to be able to access its system controller.
Put in a documentation blurb for this.

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/arm/arm-boards

@@ -93,6 +93,14 @@ Required nodes:
   a core-module with regs and the compatible strings
   "arm,core-module-versatile", "syscon"
 
+Optional nodes:
+
+- arm,versatile-ib2-syscon : if the Versatile has an IB2 interface
+  board mounted, this has a separate system controller that is
+  defined in this node.
+  Required properties:
+  compatible = "arm,versatile-ib2-syscon", "syscon"
+
 ARM RealView Boards
 -------------------
 The RealView boards cover tailored evaluation boards that are used to explore