Add tf-psa-crypto module

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/.gitmodules b/.gitmodules
index 4fb26b5..4612b3d 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,6 @@
 [submodule "framework"]
 	path = framework
 	url = https://github.com/Mbed-TLS/mbedtls-framework
+[submodule "tf-psa-crypto"]
+	path = tf-psa-crypto
+	url = https://github.com/Mbed-TLS/TF-PSA-Crypto.git