modules: tf-m: fix innocuous build warning

This removes the TF-M CMake warning "Actual TF-M version is not
available from Git repository.  Settled to v2.1.0" which always
got triggered because Zephyr's TF-M repo does not have the upstream
Git tags.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>