Necessary fix in drivers to upgrade to mbedtls-2.18.0

Include x509.h header file explicitly. Update docs.

Change-Id: If2e52c2cd3056654406b7b6779b67eea5cc04a48
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>

docs/getting_started/prerequisites.rst

@@ -60,7 +60,7 @@ supporting tools:
 
 The following libraries are required for Trusted Board Boot support:
 
-- mbed TLS == 2.16.2 (tag: ``mbedtls-2.16.2``)
+- mbed TLS == 2.18.0 (tag: ``mbedtls-2.18.0``)
 
 These tools are optional:
 

drivers/auth/cryptocell/712/cryptocell_crypto.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -22,6 +22,7 @@
 #include <lib/utils.h>
 
 #include <mbedtls/oid.h>
+#include <mbedtls/x509.h>
 
 #define LIB_NAME		"CryptoCell 712 SBROM"
 #define RSA_SALT_LEN		32

drivers/auth/mbedtls/mbedtls_crypto.c

@@ -12,6 +12,7 @@
 #include <mbedtls/memory_buffer_alloc.h>
 #include <mbedtls/oid.h>
 #include <mbedtls/platform.h>
+#include <mbedtls/x509.h>
 
 #include <common/debug.h>
 #include <drivers/auth/crypto_mod.h>