crypto: blake2s - add comment for blake2s_state fields

The first three fields of 'struct blake2s_state' are used in assembly
code, which isn't immediately obvious, so add a comment to this effect.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>