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>
diff --git a/drivers/auth/mbedtls/mbedtls_crypto.c b/drivers/auth/mbedtls/mbedtls_crypto.c
index 04fbc64..4c6dc0f 100644
--- a/drivers/auth/mbedtls/mbedtls_crypto.c
+++ b/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>