hwrng: via_rng - Support VIA Nano hardware RNG on X86_64 builds

Fix Kconfig to build via-rng.ko on X86_64 builds, as the VIA Nano
CPU supports x86_64, too.

Signed-off-by: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>