redoing it as a separate driver. Now that each hardware timer is handled by
a separate instance of the timer driver, it no longer makes sense to bundle
the pps driver with the regular timecounter code. (When all 8 timers were
handled by one driver there was no choice about this.)
Split the hardware register definitions out to their own file, so that the
new pps driver (coming in a separate commit later) can share them.
With the PPS driver gone, the question of which hardware timer to use for
what purpose becomes much easier (some instances can't do the PPS capture).
Now we can just hardcore timer2 for eventtimer and timer3 for timecounter.
This also now only instantiates devices for the 2 hardware timers actually
used to implement eventtimer and timecounter. This is required so that
other drivers can come along and attach to other hardware timers to provide
other functionality. (In addition to PPS, this hardware can also do PWM
stuff, general pulse width and frequency measurements, etc. Maybe some
day we'll have drivers for those things.)
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