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>
diff --git a/library/psa_crypto_aead.h b/library/psa_crypto_aead.h
index fcac5ca..ef4842e 100644
--- a/library/psa_crypto_aead.h
+++ b/library/psa_crypto_aead.h
@@ -263,6 +263,8 @@
  * \retval #PSA_ERROR_NOT_SUPPORTED
  *         Algorithm previously set is not supported in this configuration of
  *         the library.
+ * \retval #PSA_ERROR_INSUFFICIENT_MEMORY
+ *         (GCM and CCM only) Unable to allocate buffer for nonce.
  */
 psa_status_t mbedtls_psa_aead_set_nonce(
     mbedtls_psa_aead_operation_t *operation,