hwrng: via - support new Centaur CPU

New Centaur CPU(Family > 6) supprt Random Number Generator, but can't
support MSR_VIA_RNG. Just like VIA Nano.

Signed-off-by: David Wang <davidwang@zhaoxin.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>