feat(realm): add MbedTLS as submodule

This patch adds MbedTLS repository as a git submodule.
The version of MbedTLS added is v3.6.2.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I744ef44e61c85ade3d56920cfc1b8fd1a27bb045
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..1946491
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "ext/mbedtls"]
+	path = ext/mbedtls
+	url = https://github.com/Mbed-TLS/mbedtls.git
diff --git a/docs/getting_started/obtain.rst b/docs/getting_started/obtain.rst
index cffab2d..68b4072 100644
--- a/docs/getting_started/obtain.rst
+++ b/docs/getting_started/obtain.rst
@@ -5,7 +5,9 @@
 
 ::
 
-    git clone https://git.trustedfirmware.org/TF-A/tf-a-tests.git
+    git clone --recursive https://git.trustedfirmware.org/TF-A/tf-a-tests.git
+
+Note that TF-A-Tests will also fetch the MbedTLS repo as a git submodule.
 
 --------------
 
diff --git a/docs/getting_started/requirements.rst b/docs/getting_started/requirements.rst
index 553975f..14e4458 100644
--- a/docs/getting_started/requirements.rst
+++ b/docs/getting_started/requirements.rst
@@ -16,6 +16,18 @@
 distributions should also work fine, provided that the tools and libraries
 can be installed.
 
+Dependencies
+------------
+
+This section lists the dependencies for TF-A-Tests which are added as
+as a git submodule.
+
+======================== =====================
+        Name             Version
+======================== =====================
+Mbed TLS                 3.6.2
+======================== =====================
+
 Toolchain
 ---------
 
diff --git a/ext/mbedtls b/ext/mbedtls
new file mode 160000
index 0000000..107ea89
--- /dev/null
+++ b/ext/mbedtls
@@ -0,0 +1 @@
+Subproject commit 107ea89daaefb9867ea9121002fbbdf926780e98