Establish interrupts for both keyboard and mouse slots at pckbc attach time,

Establish interrupts for both keyboard and mouse slots at pckbc attach time,
rather than lazily from pckbc when slots are discovered. This is consistent
with what other isa devices (and pckbc on non-isa busses) do, and as a side
effect, this makes the dmesg output shorter.

This will also let us get rid of pckbc's intr_establish() callback in a
later diff.

Prompted by krw@ noticing ugly kernel output in a configuration with the
mouse slot left empty. ok krw@ mpi@