chore: move mbedtls into contrib

Align with other submodules so they all live in the same subdirectory.

Change-Id: I784d69f4473fa06fea7e0c1ec0d53abf2c8ca12f
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/.gitmodules b/.gitmodules
index 844ca3c..a5a55ef 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,5 +1,5 @@
 [submodule "ext/mbedtls"]
-	path = ext/mbedtls
+	path = contrib/mbedtls
 	url = https://github.com/Mbed-TLS/mbedtls.git
 [submodule "libtl"]
 	path = contrib/libtl