soc: stm32wb0: replace SYS_INIT with early init hook

STM32WB0 series was missed when SoC initialization code was migrated
from SYS_INIT routines to the new soc_early_init_hook method
(c.f. commit c6a03606)

Update that series' initialization code to align it with all others.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>