Unify cpu_init for all Kinetis CPUs to reduce code duplication.
Updated the MCG driver implementation to make the configuration easier.
Most clock settings are initialized by kinetis_mcg_init() called from
cpu_init. Board specific external clock source initialization
(FRDM-K64F, PhyNode) needs to be performed in board_init instead of
in cpu_init.
The new implementation uses a precalculated map of which mode to switch
to next if going from mode A to mode B. This simplifies the
implementation for moving between modes which are not direct neighbors.
See mcg.h documentation for a diagram of the state machine for the
clocking modes. Also found in the CPU reference manual of all Kinetis
CPUs, MCG chapter, MCG mode state diagram.
