Add an interface to handle interrupt controllers that have a contiguous
range of interrupts they pass to a second controller driver to handle.
The parent driver is expected to detect when one of these interrupts has
been triggered and call intr_child_irq_handler to pass the interrupt to
a child. The children controllers are then expected to manage the range
by allocating interrupts as needed.

This will initially be used by the ARM GICv3 driver, but is is expected to
be useful for other driver where this type of allocation applies.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6436