Clock: Common: constants now use Hz as a base unit
Clock: Common: added constexpr to convert frequencies

If e.g. f is to contain a frequency value of MHz1 (=1 000 000)
and you need the value in kHz (=> 1 000) you can write:
int f = MHz1;          // 1 000 000
int f_in_kHz = kHz(f); // 1 000