Update .gitmodules file to point to a sibling branch in mbed-crypto

diff --git a/.gitmodules b/.gitmodules
index 33ccb13..332660c 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "crypto"]
 	path = crypto
 	url = https://github.com/ARMmbed/mbed-crypto
-	branch = development
+	branch = iotssl-2697-psa-key-allocation-crypto