hwrng: ppc4xx - add support for ppc4xx TRNG

Various PowerPC 4xx SoCs contain a TRNG embedded in the Security function.
This adds a device driver for that TRNG.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>