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>