Platform: Initialize PSA test memory on power-on and pin reset

Initialize the memory area on both power-on and pin reset
Only initialize memory from the non-secure application to prevent
initializing twice
Clear reset register after reading

Signed-off-by: Andreas Vibeto <andreas.vibeto@nordicsemi.no>
Change-Id: If3aee8d5b31f0a48ec432e3d3f39c029cda3fd7a