Autoconfiguration cleanup for arckbd(4):
- Use device_t and cfdata_t rather than explicit structure pointers.
- Split device and (non-existent) softc for arcwskbd and arcwsmouse.
- Use aprint_*() rather than printf().
- Use device_*() to access fields of device_t.
- Use device_private() rather than a cast to convert from device_t to softc.
device/softc split not done for arckbd(4) itself because there are lots
of void* cookies that get treated as arckbd_softc at one end and device_t
at the other, and they'll need a little thought to clean up.

(bjh21)