hwrng: cavium - Add Cavium HWRNG driver for ThunderX SoC.

The Cavium ThunderX SoC has a hardware random number generator.
This driver provides support using the HWRNG framework.

Signed-off-by: Omer Khaliq <okhaliq@caviumnetworks.com>
Signed-off-by: Ananth Jasty <Ananth.Jasty@cavium.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>