ARM: ixp4xx: Add DT bindings

This adds initial device tree bindings for the IXP4xx machines.
This time I tried something wild and crazy and try to make proper
JSON-style YAML bindings for the top level.

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