boards: nrf9160 DK: update board documentation for TF-M

Minor updates to the documentation of the nRF9160 DK
board, stressing that TF-M is the default Secure
firmware binary for non-secure nRF9160 builds.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
1 file changed