Add support for the MIPS74K SoC family performance counters events.

These are similar to the mips24k performance counters - some are
available on perfcnt0/3, some are available on perfcnt1/4.
However, the events aren't all the same.

• Add the events, named the same as from Linux oprofile.
• Verify they're the same as "MIPS32(R) 74KTM Processor Core Family Software User's Manual"; Document Number: MD00519; Revision 01.05.
• Rename INSTRUCTIONS to something else, so it doesn't clash with the alias INSTRUCTIONS. I'll try to tidy this up later; there are a few other aliases to add and shuffle around.

Tested:

• QCA9558 SoC (AP135 board) - MIPS74Kc core (no FPU.)
• make universe; where it didn't fail for other reasons.

TODO:

• It'd be nice to support the four performance counters in at least this hardware, rather than just two.

Reviewed by: bsdimp ("looks good; don't break world".)