Kinetis:Add the configuring SIM_CLKDIV2[USBFRAC, USBDIV] in kinetis_clockconfig

  If a board.h provides BOARD_SIM_CLKDIV2_FREQ it will configure the
  SIM_CLKDIV2 based on the additional provided
  BOARD_SIM_CLKDIV2_USBFRAC and BOARD_SIM_CLKDIV2_USBDIV

  The reason for doing this globaly is that the output the
  SIM_CLKDIV2 divisor may be also used for other IP blocks in
  future configurations (as is done for SIM_CLKDIV3)