Set default refspec value as NULL for dependency repos

TF-M dependency versions can be automatically parsed by CI scriptsg from
CMake system now. So keep default refspec value as NULL to use the TF-M
specified dependency version by default.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I91c4ed7f5a4effb154054f8aeb43f6dccf0ecd9b
14 files changed