specify submodule branches

Author: microbit-mark

.gitmodules

@@ -1,9 +1,12 @@
 [submodule "software/spec-i2c-protocol"]
 	path = software/spec-i2c-protocol
 	url = https://github.com/microbit-foundation/spec-i2c-protocol
+	branch = main
 [submodule "software/spec-power-mangement"]
 	path = software/spec-power-mangement
 	url = https://github.com/microbit-foundation/spec-power-management
+	branch = main
 [submodule "software/spec-universal-hex"]
 	path = software/spec-universal-hex
 	url = https://github.com/microbit-foundation/spec-universal-hex
+	branch = master