rp: add generic dormant-sleep functionality

this is "generic" in that it doesn't require the user to set up anything
specific to go to dormant sleep, unlike the C sdk which requires clock
sources to be configured explicitly and doesn't much care about PLLs. we
will instead take a snapshot of the current clock configuration, switch
to a known clock source (very slow rosc, in this case), go to sleep, and
on wakeup undo everything we've done (ensuring stability of PLLs and
such).

tested locally, but adding tests to HIL seems infeasible. we'd need at
least another pico or extensive modifications to teleprobe since
dormant-sleep breaks SWD (except to rescue-dp), neither of which is
feasible at this point. if we *did* want to add tests we should check
for both rtc wakeups (with an external rtc clock source) and gpio wakeups.

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