modules: tfm: Fix cmake warnings for unused variables

Fix cmake warnings for TF-M build system when PSA tests are not enabled.
This produces the following warnings:
"
CMake Warning:
  Manually-specified variables were not used by the project:

    PSA_TOOLCHAIN_FILE
    TOOLCHAIN
"

These warnings can point to cache or config variables being updated in
the TF-M build system so keep this free of warnings.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>