crypto: ixp4xx - Add DT bindings

This adds device tree bindings for the ixp4xx crypto engine.

Cc: Corentin Labbe <clabbe@baylibre.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>