hwrng: amd - Access hardware via ioread32/iowrite32

Instead of accessing hw directly via pmbase, it's better to
access after ioport_map() via ioread32/iowrite32.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>