Sensor model needs expansion for multichannel sensors

Currently, the way sensors -- which in practice right now means "PMBus sensors" -- are defined assumes that a given sensor device has a single output channel of each kind. Voltage, power, current, temperature, etc.

This kind of works, but doesn't describe all devices, and in particular doesn't describe all devices we are currently shipping.

This is a placeholder issue to remind us of this. Ideally we would be able to have multiple named channels of each unit type per device, so we could distinguish e.g. "die temperature" from "cpu0 temperature" (hypothetically, not a real device) or "power in" from "power out" on the RAA229618 (which is real and can be important for computing overhead/loss).

The issue names no files, tests, or entry points. Start by locating the current PMBus sensor model and reviewing the shipped devices that need multiple outputs; done should support multiple named channels for each unit type, including distinctions such as die versus CPU temperature and power input versus output.