regulator: Add some brief design documentation

Regulator API design notes

==========================

This document provides a brief, partially structured, overview of some

of the design considerations which impact the regulator API design.

Safety

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 - Errors in regulator configuration can have very serious consequences

   for the system, potentially including lasting hardware damage.

 - It is not possible to automatically determine the power confugration

   of the system - software-equivalent variants of the same chip may

   have different power requirments, and not all components with power

   requirements are visible to software.

  => The API should make no changes to the hardware state unless it has

     specific knowledge that these changes are safe to do perform on

     this particular system.

Consumer use cases

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 - The overwhelming majority of devices in a system will have no

   requirement to do any runtime configuration of their power beyond

   being able to turn it on or off.

 - Many of the power supplies in the system will be shared between many

   different consumers.

  => The consumer API should be structured so that these use cases are

     very easy to handle and so that consumers will work with shared

     supplies without any additional effort.