Properly handle GCM's range of nonce sizes

Add comment to the effect that we cannot really check nonce size as the
GCM spec allows almost arbitrarily large nonces. As a result of this,
change the operation nonce over to an allocated buffer to avoid overflow
situations.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/psa_crypto.c

@@ -3429,6 +3429,12 @@ psa_status_t psa_aead_set_nonce( psa_aead_operation_t *operation,
         goto exit;
     }
 
+    /* Not checking nonce size here as GCM spec allows almost abitrarily large
+     * nonces. Please note that we do not generally recommend the usage of
+     * nonces of greater length than PSA_AEAD_NONCE_MAX_SIZE, as large nonces
+     * are hashed to a shorter size, which can then lead to collisions if you
+       encrypt a very large number of messages. */
+
     status = psa_driver_wrapper_aead_set_nonce( operation, nonce,
                                                 nonce_length );