bhnd(4): implement MIPS and PCI(e) interrupt support
On BHND MIPS SoCs, this replaces the use of hard-coded MIPS IRQ#s in the
common bhnd(4) core drivers; we now register an INTRNG child PIC that
handles routing of backplane interrupt vectors via the MIPS core.
On BHND PCI devices, backplane interrupt vectors are now routed to the
PCI/PCIe host bridge core when bus_setup_intr() is called, where they are
dispatched by the PCI core via a host interrupt (e.g. INTx/MSI).
The bhndb(4) bridge driver tracks registered interrupt handlers for the
bridged bhnd(4) devices and manages backplane interrupt routing, while
delegating actual bus interrupt setup/teardown to the parent bus on behalf
of the bridged cores.
Approved by: adrian (mentor, implicit)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12518