Add support for directoryName subjectAltName

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/include/mbedtls/x509.h b/include/mbedtls/x509.h
index bd1947e..82cffff 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -294,7 +294,8 @@
     int type;                              /**< The SAN type, value of MBEDTLS_X509_SAN_XXX. */
     union {
         mbedtls_x509_san_other_name other_name; /**< The otherName supported type. */
-        mbedtls_x509_buf   unstructured_name; /**< The buffer for the unconstructed types. Only rfc822Name, dnsName and uniformResourceIdentifier are currently supported */
+        mbedtls_x509_name directory_name;
+        mbedtls_x509_buf unstructured_name; /**< The buffer for the unstructured types. rfc822Name, dnsName and uniformResourceIdentifier are currently supported. */
     }
     san; /**< A union of the supported SAN types */
 }