soc: arm: st_stm32 add low power to stm32wb series

This patch introduces the support of low power modes
for the STM32WBxx from STMicroelectronics based on the lptim
Here, the power modes are sleep modes have lptimer as wakeup source.
The sleep modes are configured by the SYS_POWER_MANAGEMENT.

Signed-off-by: Francois Ramu <francois.ramu@st.com>