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