Coldfire: M5271: Allow board header file to specify clock multiplier

M5271 dynamic clock multiplier. It is currently fixed at 100MHz.

Allow the board header file to set their own multiplier and divider.
Added the #define for the multiplier and divider to the cpu header file.

Signed-off-by: Richard Retanubun <RichardRetanubun@RuggedCom.com>