hwrng: pixocell - add support for picoxcell TRNG

This driver adds support for the True Random Number Generator in
the Picochip PC3X3 and later devices.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>