plumb power controller event history for upstack consumers

[wesolows]

As part of an automated (or, initially, manual) response to various fault conditions, it would be useful to obtain fault and other event status from power controllers. These include the complex blackbox data available in Renesas multiphase controllers as well as the simpler event ring buffers in devices like the MAX5970. While it is generally possible to obtain several classes of this data via Humility with a probe, production-usable plumbing is needed to obtain this data in the field and return it as part of service bundles (oxidecomputer/omicron#2235).