exposing pac items kind of undermines the unstable-pac feature. directly
exposing register structure is also pretty inconvenient since the clock
switching code takes care of the src/aux difference in behavior, so a
user needn't really be forced to write down decomposed register values.
the datasheet says that the xosc may be run by feeding a square wave
into the XIN pin of the chip, but requires that the oscillator be set to
pass through XIN in that case. it does not mention how, the xosc
peripheral does not seem to have any config bits that could be set to
this effect, and pico-sdk seems to have no (or at least no special)
handling for this configuration at all. it can thus be assumed to either
be not supported even by the reference sdk or to not need different
handling.
solvers usually output fbdiv directly, using vco_freq to get back to
fbdiv is not all that necessary or useful. both vco_freq and fbdiv have
hidden constraints, but vco_freq is a lot less accurate because the
fbdiv value resulting from the division may be off by almost a full
ref_freq's worth of frequency.
also fixes the usb pll config, which ran the pll vco way out of (below)
spec.
we might not configure both, so we should put the others into reset
state. leaving them fully as is might leave them running, which might
not be the goal for runtime reconfig (when it comes around). this now
mirrors how we reset all clock-using peripherals and only unreset those
that are properly clocked.
this is only really useful for runtime *re*configuration, which we don't
currently support. even runtime reconfig probably won't need it, unless
we keep taking the sledgehammer approach of reconfiguring everything all
the time.
