soc: st: stm32: stm32c0x: Add stop mode support

Enables low power stop mode for C0.

Code is taken from F4 family, tested on nucleo-c71rb with
samples/basic/blinky.

Power consumption in run mode 3.7 mA, in stop mode ~87 uA.

Signed-off-by: Tomáš Juřena <jurenatomas@gmail.com>