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>

include/crypto/blake2s.h

@@ -24,6 +24,7 @@ enum blake2s_lengths {
 };
 
 struct blake2s_state {
+	/* 'h', 't', and 'f' are used in assembly code, so keep them as-is. */
 	u32 h[8];
 	u32 t[2];
 	u32 f[2];