Begin the MIPS_INTRNG support.

Begin the MIPS_INTRNG support.

This is a prelude to intr-ng support for MIPS boards that need it -
notably the CI20 port from kan@ that's upcoming, but also work that
Stanislav is doing for the Mediatek platforms.

This is the initial platform dependent bits in include/intr.h, some
#defines for the nexus code for the intrng initialisation/runtime
bits, some changed naming (which I'll fix later to be the same, much
like what I did for ARM intr-ng) in exception.S, and the first cut
at a PIC.

Stanislav and I refactored out the common code for intrng support,
so the mips intrng definitions are quite small (sys/mips/include/intr.h.)

This is all work done by kan@, which stanislav has been cherry picking
into common code for his mediatek chipset work.

Tested:

* Carambola2 - no regressions (not intr-ng though!)

Submitted by:	Stanislav Galabov <sgalabov@gmail.com>
Reviewed by:	kan (original author)
Differential Revision:	https://reviews.freebsd.org/D5182