crypto: rng - RNG interface and implementation

This patch adds a random number generator interface as well as a
cryptographic pseudo-random number generator based on AES.  It is
meant to be used in cases where a deterministic CPRNG is required.

One of the first applications will be as an input in the IPsec IV
generation process.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>