modules: tf-m: fix git clone during build

As of v2.1.0 TF-M has updated to CMSIS v6 and switched from hosting
the sources to depending on the upstream repository, cloning it at
build time.

To prevent a download from happening during the build, CMSIS v6 sources
are pushed to Zephyr's fork of TF-M and the `CMSIS_PATH` CMake variable
is set to point to them.

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