Update MbedTLS to 2.26.0

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Idfa92383d03f70bb7af864b7b8d35ca66102cbca
diff --git a/tf-m-build-docs.yaml b/tf-m-build-docs.yaml
index 47d9fbb..4b7fec8 100644
--- a/tf-m-build-docs.yaml
+++ b/tf-m-build-docs.yaml
@@ -59,7 +59,7 @@
           default: 'master'
       - string:
           name: 'MBEDTLS_VERSION'
-          default: 'mbedtls-2.25.0'
+          default: 'mbedtls-2.26.0'
       - string:
           name: 'MBEDTLS_URL'
           default: 'https://git.trustedfirmware.org/mirror/mbed-tls.git'