crypto: sun8i-ss - Add more comment on some structures

This patch adds some comment on structures used by sun8i-ss.
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/allwinner/sun8i-ss/sun8i-ss.h

@@ -195,6 +195,8 @@ struct sun8i_cipher_req_ctx {
  * @keylen:		len of the key
  * @ss:			pointer to the private data of driver handling this TFM
  * @fallback_tfm:	pointer to the fallback TFM
+ *
+ * enginectx must be the first element
  */
 struct sun8i_cipher_tfm_ctx {
 	struct crypto_engine_ctx enginectx;