Improve FREEDOM-KL25Z board pin initialization

Improve initialization for the FRDM-KL25Z board and the KL25Z128VLH4 MCU.

- Add pin name comments in pal_default_config.

- Fix FRDM-KL25Z SWD_CLK/SWD_DIO/RESET pin defaults.
  My debugger could not function when they were set to GPIO function.

- Don't let unused Hi-Z digital inputs be floating.

- Use UNCONNECTED constant to identify pins that don't exist on the MCU.