Follow the recent pckbc@isa changes and always establish all the necessary

Follow the recent pckbc@isa changes and always establish all the necessary
interrupts at pckbc attach time, and get rid of the `intr_establish'
pckbc callback.

Tested on hppa (gsckbc) and sgi (pckbc@hpc); not tested on sparc64 (pckbc@ebus)
but this attachment was already behaving this way and its intr_establish
callback was an empty function.