hw_random: ixp4xx: Turn into a module

Instead of just initializing always, which will invariably
create problems on multiplatform builds, turn this driver into
a module and pass a resource with the memory location.
The device only exist on the IXP46x so we only register
the device for the IXP46x SoC.

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