Crypto library: Migrate support to MbedTLS v2.24.0

This patch migrates the mbedcrypto dependency for TF-A
to mbedTLS repo v2.24.0 which is the latest release tag.
The relevant documentation is updated to reflect the
use of new version.

Change-Id: I116f44242e8c98e856416ea871d11abd3234dac1
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 13e25cd..91ecdf3 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -60,7 +60,7 @@
 
 The following libraries are required for Trusted Board Boot support:
 
-- mbed TLS == 2.18.0 (tag: ``mbedtls-2.18.0``)
+- mbed TLS == 2.24.0 (tag: ``mbedtls-2.24.0``)
 
 These tools are optional: