Kinetis:Refactor you use SIM_SOPT2_PLLFLLSEL, added warning

  The warning has been added because: SIM_SOPT2_PLLFLLSEL
  is a clock selection that may feed many clock subsystem:
   USB, TPM, SDHCSRC, LPUARTSRC. Therefore, there needs to
  be a global board level setting to select  the source for
  SIM_SOPT2_PLLFLLSEL and then  derive all the  sub selections
  and proper fractions/divisors for each modules clock.