hwrng: Enable COMPILE_TEST for more drivers

There's quite a few hwrng drivers which are easily enabled for
COMPILE_TEST, so let's enable them.

The dependency on HW_RANDOM is redundant, so drop that while we're here.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>