854: Implement IWDG timeout calculation r=Dirbaio a=chemicstry
Allow specifying `IndependentWatchdog` timeout as `Duration` instead of prescaler value.
Since IWDG is clocked from LSI, which differs between families, I standardized HSI/LSI definitions in RCC and used that.
Co-authored-by: chemicstry <chemicstry@gmail.com>