Read-Only IPMI sensors

[Adriana Kobylak]

There are a couple IPMI sensors that are not yet included in the 
data-driven design of IPMI because they're intended to be read-only 
(example: https://github.com/openbmc/openbmc/issues/2154).

Some options to implement these:

1. Add support in sdbusplus for read-only properties and map these 
sensors to a read-only d-bus property.

2. Have a (json?) file with the sensor settings and have IPMI and any 
other interested apps read the file directly to retrieve the sensor 
values.

3. Add read-only sensors to a d-bus object and override the "setter" 
function to be a no-op to prevent the value from being modified.

4. Combination of above / other options.

Thoughts?