soc/arm: stm32wb: Default C2 power mode to SHUTDOWN

Low power modes entry on stm32wb depends on requests coming from both
cores, with no consideration of the fact that C2 is booted  or not.

By default, set C2 power mode to shutdown at C1 start up.
If required, it will be updated by C2.
In case C2 is not started, this will allow C1 to enter any power mode
with no dependency on C2.

Fixes #38173

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>