soc: silabs: Fix the PRIMASK for Silabs S2 SoCs

sl_power_manager_sleep() doesn't expect the PRIMASK to be set when called.
Setting BASEPRI to 0 was moved to sl_power_manager_is_ok_to_sleep(),
this function is called after sl_power_manager_sleep() has set the PRIMASK.
Added sli_power_manager_on_wakeup() to force a clock restore before the
interrupt are handled. Added a call to retrieve the startup measurements,
reducing the early wakeup time.

Signed-off-by: Bastien Beauchamp <bastien.beauchamp@silabs.com>