soc: stm32f1x: Add support for stop/standby modes

Add config and overlay to samples for testing stop/standby modes:

- samples/boards/st/power_mgmt/blinky
- samples/boards/st/power_mgmt/wkup_pins

I've measured consumption for each low-power mode:
- stop (regulator in run mode) ~217 uA
- stop (regulator in low-power mode) ~206 uA
- standby mode ~3.5 uA

Low-power mode wakeup timings from the datasheet,
but it barely meets these in reality:
- stop (regulator in run mode) 3.6 us
- stop (regulator in low-power mode) 5.4 us
- standby 50 us

It's possible to use RTC as idle timer to exit from stop mode.

Signed-off-by: Oleh Kravchenko <oleg@kaa.org.ua>
10 files changed