[WIP] Freescale Kinetis K20

This is my reorganized and rebased K20 work which still has yet to be tested. Unfortunately many of the K20's peripherals are hold-overs from the 8-bit days, so I've had  to generalize the ioreg macros to work over arbitrary register types. Otherwise the changes are pretty straightforward.

Again, this is untested but I thought it would be a good idea to at least advertise that the code exists.
Please fix here and other files. You must "own" any file you created in the tree (that actually makes sense for me, e.g. in an unlikely case when I would decide to change the license, I will have to contact all the contributors). In case if your file is mostly (> 90%) based on other file, you can mention the original author on the next line (like that is gone in [linux kernel](https://github.com/torvalds/linux/blob/master/arch/arm/mach-pxa/palmtx.c) for example)