arm-safety-island: Introduce common overlay file

This commit adds a new common overlay file to consolidate
shared configurations and define minimum memory requirements.
This ensures that essential requirements are consistently
maintained across different application overlays, preventing
unintended overwrites or conflicts with other overlays.

Additionally, I enabled INIT_STACKS to mitigate the issues raised
during the FVP reboot. This applies to the stack areas for threads,
as well as the interrupt stack.

Signed-off-by: Divin Raj <divin.raj@arm.com>