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

Three compile-time constants are used:

	CFG_866_OSCCLK            - input quartz clock
	CFG_866_CPUCLK_MIN        - minimum allowed CPU clock
	CFG_866_CPUCLK_MAX        - maximum allowed CPU clock
	CFG_866_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 note that for now the new clock-handling code has been enabled
for the TQM866M board only, even though it should be pretty much
common for other MPC859 / MPC866 based boards also. Our intention
here was to move in small steps and not to break the existing code
for other boards.