updates to evbmips, new common mips3 code

All,

I've moved a significant amount of the clock interrupt and related
functionality from evbmips to a common file, arch/mips/mips/mips3_clock.c

This means that ports which want to use the CP0 counter register for
their system clock can just use this, and remove a lot of MD code for
it.  Look at evbmips for an example.  Of particular note, you might want
to look at the before/after for evbmips/evbmips/interrupt.c, because
that shows how to rip out the clock interrupt handler and replace with a
call to mips3_clockintr().

Using this code also gives, for free, a MIPS3 CP0-count based
timecounter.  So if you do this, you can easily switch to
__HAVE_TIMECOUNTER.

I'd be happy to help convert a MIPS3 based port that already uses the
CP0 counter as the system clock, if someone wants to volunteer to test. 
I don't have hardware, so I can only compile-test anything other than
evbmips.

-- 
Garrett D'Amore, Principal Software Engineer
Tadpole Computer / Computing Technologies Division,
General Dynamics C4 Systems
http://www.tadpolecomputer.com/
Phone: 951 325-2134  Fax: 951 325-2191