soc/arm/silabs_exx32/common/soc_power: add missing `__weak` symbol

This commit adds a missing `__weak` symbol to the `pm_policy_next_state`
function.

This is needed for e.g. `tests/kernel/profiling/profiling_api` to build
correctly.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>