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 CPU has multiple issues and several parts of the platform code
does not even compile cleanly for this CPU in the current state.
This removes support for parts MK60DN256ZVLL10, MK60DN512ZVLL10
(note the Z) CPUs with this part number were used in Mulle v0.60 which
only has been used in some in-house projects at Eistec and LTU.
