soc: silabs: Add soc_prep_hook() for Series 2

CMSIS SystemInit is not used in Zephyr. Implement the functionality
that isn't already done by Zephyr startup using soc_prep_hook().

The reason the lack of TrustZone init did not create immediately obvious
issues previously is that SMU faults can only happen if the SMU clock is
enabled.

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