Specialized peripheral PIC drivers may need to handle most interrupt
setup steps themselves without touching newbus. Two cases are
intr_add_handler() and intr_describe() which are internally used by
intr_setup_irq()/intr_describe_irq(). Exposing these allow for
alternative use case.
In fact the BUS interface for INTRNG is arguably a layering violation.
Those 6 functions could just as well be in nexus.c or a kernel library.