ivory-bsp-stm32: fix pllcfgr bug in init, other bugs fixed

- chip was prev running at 56mhz instead of 168, because i used the
  literal (fromRep 2) rather than _div2 constructor
- maybe see if bitdata's types can prevent fromRep from working
  when constructors exist?
ivory-bsp-stm32: rcc getfreq now uses constants rather than runtime calc

- todo, factor getfreq and boardhse typeclass to external definition, init
  clock setup depend on that instead