crypto: sha512 - Expose generic sha512 routine to be callable from other modules

Other SHA512 routines may need to use the generic routine when
FPU is not available.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Conflicts:
	crypto/sha512_generic.c
	include/crypto/sha.h