pm: add power management for stm32f4x

Add soc power management for the STM32F4x chips.

One low power state is added supported by all chips from the family -
the Stop mode with voltage regulator in low-power mode.

The Stop mode for STM32F chips has to work with the IDLE timer -
CORTEX_M_SYSTICK_IDLE_TIMER, because PLL and HSI are disabled in the
Stop mode (Systick is not clocked). The only possible wakeup source is
RTC, which works as a IDLE timer for the Systick.

The exit latency may need to be adjusted per system, depending on the
system tick frequency and other variables.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>