hw_random: ixp4xx: Add OF support

This makes the hardware random number generator found in
the IXP46x SoCs probe from the device tree.

Cc: Deepak Saxena <dsaxena@plexity.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>