soc: silabs: s2: Only configure SMU and SAU once on boot

If a bootloader is used, it will configure the SMU and SAU.
The application cannot also configure it, since it can't know
what alias (S vs NS) to access the CMU through. Only connect
the interrupt to the application's vector table if a bootloader
is present.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>