dt-bindings: anybus-controller: document devicetree binding

This patch adds devicetree binding documentation for the
Arcx anybus controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>