crypto: DRBG - switch to HMAC SHA512 DRBG as default DRBG

crypto: DRBG - switch to HMAC SHA512 DRBG as default DRBG

The default DRBG is the one that has the highest priority. The priority
is defined based on the order of the list drbg_cores[] where the highest
priority is given to the last entry by drbg_fill_array.

With this patch the default DRBG is switched from HMAC SHA256 to HMAC
SHA512 to support compliance with SP800-90B and SP800-90C (current
draft).

The user of the crypto API is completely unaffected by the change.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Acked-by: simo Sorce <simo@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>