submodules: Add new submodule 3rdparty/cmocka

Cmocka unit testing framework is used for writing and building coreboot
unit tests. This repo will be checked-in only when building some test
targets.

Signed-off-by: Jan Dabros <jsd@semihalf.com>
Change-Id: I3cdfd32f5bba795d5834ebeae1afff0f7006a0d1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41652
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

.gitmodules

@@ -42,3 +42,7 @@
 	url = ../amd_blobs
 	update = none
 	ignore = dirty
+[submodule "3rdparty/cmocka"]
+	path = 3rdparty/cmocka
+	url = ../cmocka.git
+	update = none