nrf/drivers/ticker: Rework ticker functions for microbit display/music.

- Rename init function to ticker_init0.
- Implement ticker_register_low_pri_callback (recycle of unused
  set_low_priority_callback function which was unimplemented).
- Add support for registering 2 low pri callbacks.  For now, one intended
  for microbit display, and one for modmusic.