Postpone allocation of IRQ resource to the time when interrupt
controller devices are attached. This has already been done for
bus_setup_intr().

There was no doubt that if someone wants to setup an interrupt,
corresponding interrupt controller device must already be attached.
However, the same must be valid for allocation of an interrupt resource
unless the allocation is done blindly, without any information that
such interrupt even exists. While it was done this blind way before,
it won't be possible after next INTRNG change.