Upgrade mbed TLS version

This patch updates the User Guide to recommend the latest version
of mbed TLS library to use with ARM Trusted Firmware.

 - Upgrade mbed TLS library:   2.2.1 -> 2.4.2

Change-Id: Ifb5386fec0673d6dbfdaa474233e397afc279c85

docs/user-guide.md

@@ -799,7 +799,7 @@ images with support for these features:
     modules by checking out a recent version of the [mbed TLS Repository]. It
     is important to use a version that is compatible with TF and fixes any
     known security vulnerabilities. See [mbed TLS Security Center] for more
-    information. This version of TF is tested with tag `mbedtls-2.2.1`.
+    information. The latest version of TF is tested with tag `mbedtls-2.4.2`.
 
     The `drivers/auth/mbedtls/mbedtls_*.mk` files contain the list of mbed TLS
     source files the modules depend upon.