The current implementation allows the user to specify the desired CPU
  clock value, in MHz, via an environment variable "cpuclk".

  Four compile-time constants are used:

  	CONFIG_8xx_OSCLK          - input quartz clock

  	CONFIG_SYS_8xx_CPUCLK_MIN        - minimum allowed CPU clock
  	CONFIG_SYS_8xx_CPUCLK_MAX        - maximum allowed CPU clock

  	CONFIG_8xx_CPUCLK_DEFAULT - default CPU clock value

  
  If the "cpuclk" environment variable value is within the CPUCLK_MIN /
  CPUCLK_MAX limits, the specified value is used. Otherwise, the
  default CPU clock value is set.

  Please make sure you understand what you are doing, and understand
  the restrictions of your hardware (board, processor). For example,
  ethernet will stop working for CPU clock frequencies below 25 MHz.

  Please note that the new clock-handling code is enabled if
  CONFIG_8xx_CPUCLK_DEFAULT is defined. Since this mechanism supports
  only MPC866 and newer CPUs, this constant MUST NOT be defined for
  MPC823/850/860/862 series. The clock generation algorithm for older
  chips is different and has not been implemented yet. If you need it,
  your patch is welcome.