Rework cbus(4) interrupt support a bit. Instead of merging devhandle and

Rework cbus(4) interrupt support a bit.  Instead of merging devhandle and
devino into a pseudo-sysino, directly use the devino as the ihandle.  The
devhandle is stored in the cbus softc, and accessed through the bus space
tag.  This allows us to have more than 256 interrupts on a single cbus, and
avoids relying on the lower bits of the devhandle being zero.