soc: stm32l1x: Add support for sleep/stop/standby modes

Add stm32l1_disco and nucleo_l152re overlays for testing
sleep/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:
- low-power sleep ~1.72mA;
- stop mode ~324uA;
- standby mode ~2.2 uA;

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

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