tf-m: Silence harmless version warning

Silence harmless version warning for users that have tagged TF-M.

TF-M creates a very noisy warning when you tag it:

CMake Warning at cmake/version.cmake:28 (message):
  Actual TF-M version is not available from Git repository.  Settled to
  v2.0.0
Call Stack (most recent call first):
  CMakeLists.txt:22 (include)

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>