soc: stm32wl: add power management support

This adds power management support for the STM32WL series.

Suspend-to-idle is mapped to the three stop states (wake up from any
EXTI, including LPTIM), and soft-off can trigger either standby or
shutdown (wake up in reset).

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>