Add 2 files for multiplication in GF(128)

Multiplication in GF(128) is required by the AES-XEX mode for computing
X in the XEX formula from
https://en.wikipedia.org/wiki/Disk_encryption_theory#Xor-encrypt-xor_.28XEX.29

include/mbedtls/gf128mul.h

@@ -0,0 +1,46 @@
+/**
+ * \file gf128mul.h
+ *
+ * \brief Fast multiplication in GF(128)
+ *
+ *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  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.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+#ifndef MBEDTLS_GF128MUL_H
+#define MBEDTLS_GF128MUL_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * \brief          Big-Endian definition for 128 bits elements
+ */
+typedef unsigned char be128[16];
+
+/**
+ * \brief          Multiplication in GF(128):
+ *                 r = x times x^4 times x^8 in GF(2^128)
+ *
+ * \param x        the 128-bits number you want to multiply
+ * \param r        result
+ */
+void gf128mul_x_ble(be128 r, const be128 x);
+
+
+
+#endif /* gf128mul.h */