tests: arch: arm: update for MPS4 Corstone315

DT config for MPS4 corstone315 boards which can be tested with simulation.
Creates a custom section in the unused SRAM region. Executing code
in this region in privileged mode will result in an MPU fault while,
execution in unprivileged mode should work fine.

Signed-off-by: Shaunak saha <ssaha@tsavoritesi.com>