[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 take "ownership" of the FIXME: `// FIXME(bgamari): use system clock`