hwrng: ba431 - Add dependency on HAS_IOMEM

The ba431 driver depends on HAS_IOMEM and this was missing from
the Kconfig file.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 0289e9be ("hwrng: ba431 - add support for BA431 hwrng")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>