Updates and future changes on phosphor-power

[Lei YU]

This email is to introduce the current status and future changes of
phosphor-power.

## Background
Previously, [witherspoon-pfault-analysis][1] exists for Witherspoon to
provide functions like PSU monitor.
Later it's found that the code could be shared for other PSUs as long
as a property json config is provided.
So the repo is renamed to [phosphor-power][2]

## Status
Now the repo is renamed and I am adding the bitbake recipes into
meta-phosphor, see
https://gerrit.openbmc-project.xyz/c/openbmc/meta-phosphor/+/26356

However, the repo still contains something that is not generic:
* There is repo specific error defined in org/open_power/Witherspoon/
* It has a dependency to openpower-dbus-interfaces to use interfaces
in org/open_power/Sensor/Aggregation/History

That is not ideal.

## TODO
The repo specific error and the interfaces in
org/open_power/Sensor/Aggregation/History are actual generic, so they
could be moved into phosphor-dbus-interfaces to become generic
interfaces.

So I am planning to "copy" the above interfaces into
phosphor-dbus-interfaces, and when it's done, update this repo to use
the interfaces in phosphor-dbus-interfaces.

Do you have any comments or suggestions?
Thanks!

[1]: https://github.com/openbmc/witherspoon-pfault-analysis
[2]: https://github.com/openbmc/phosphor-power