soc: arm: st_stm32 add low power to stm32l4 series

This patch introduces the support of low power modes
for the STM32L4xx from STMicroelectronics based on the lptim
Here, the power modes are sleep modes with lptimer as wakeup.
Depending on the SYS_POWER_MANAGEMENT configuration.

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