crypto: crypto4xx - Avoid linking failure with HW_RANDOM=m

It is currently possible to build CONFIG_HW_RANDOM_PPC4XX=y with
CONFIG_HW_RANDOM=m which would lead to the inability of linking with
devm_hwrng_{register,unregister}. We cannot have the framework modular
and the consumer of that framework built-in, so make that dependency
explicit.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>