MIPS: Octeon: Add hardware RNG platform device.

Add a platform device for the Octeon Random Number Generator (RNG).

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>