cpu/arm/k60: Clean up UART code.

- Split off UART debug terminal code into separate module.
- Added PORT module for enabling clock gates.
- Cleaned up UART initialization and usage.
- Added debug terminal configuration parameters to config-board.h

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add missing enum values in llwu.h

This was missing when merging with the LLWU code at LTU

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>