samples: Add TF-M sample for custom secure partitions

The sample implements a dummy ARoT (Application Root of Trust) partition
in TF-M and calls it from the app.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>