bus: ixp4xx: Add DT bindings for the IXP4xx expansion bus

This adds device tree bindings for the IXP4xx expansion bus controller.

Cc: Marc Zyngier <maz@kernel.org>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>