Kinetis:Refactor clocking in kinetis_usbdev

  1) Removed SIM_CLKDIV2[USBFRAC, USBDIV] setting as it is now
     done in kinetis_clockconfig
  2) Use BOARD_USB_CLKSRC to select the clock source to the
     USB block
  3) Removed warning
  4) Removed CONFIG_TEENSY_3X_OVERCLOCK from the driver as
     the board.h will now provide BOARD_SIM_CLKDIV2_USBDIV
     and BOARD_SIM_CLKDIV2_USBFRAC to the kinetis_clockconfig