Preemptively map MIPS INTRNG interrupts on non-FDT MIPS targets

This replaces a partial workaround introduced in r305527 that was
incompatible with nested INTRNG interrupt controllers if not also using
FDT.

On non-FDT MIPS INTRNG targets, we now preemptively produce a set of fixed
mappings for the MIPS IRQ range during nexus attach. On FDT targets,
OFW_BUS_MAP_INTR() remains responsible for mapping the MIPS IRQs.

Approved by: adrian (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12385