hw_random: ixp4xx: Add DT bindings

This adds device tree bindings for the simple random number
generator found in the IXP46x SoCs.

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