Create psa_util_internal.h

Most functions in psa_util.h are going to end up there (except those
that can be static in one file), but I wanted to have separate commits
for file creation and moving code around, so for now the new file's
pretty empty but that will change in the next few commits.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/psa_util_internal.h

@@ -0,0 +1,32 @@
+/**
+ * \file psa_util_internal.h
+ *
+ * \brief Internal utility functions for use of PSA Crypto.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#ifndef MBEDTLS_PSA_UTIL_INTERNAL_H
+#define MBEDTLS_PSA_UTIL_INTERNAL_H
+
+/* Include the public header so that users only need one include. */
+#include "mbedtls/psa_util.h"
+
+#if defined(MBEDTLS_PSA_CRYPTO_C)
+
+#endif /* MBEDTLS_PSA_CRYPTO_C */
+#endif /* MBEDTLS_PSA_UTIL_INTERNAL_H */