Implemented lp run based on changing MSI speed:
https://github.com/David-OConnor/stm32-hal/blob/main/src/clocks/l_g.rs#L740
https://github.com/David-OConnor/stm32-hal/blob/main/src/low_power.rs#L26

`low_power::low_power_run(&mut clock_cfg, MsiRange::R2M, &mut dp.RCC, &mut dp.pwr)`