RFC: Support pwm in phosphor-hwmon as fan target

[Lei YU]

The current phosphor-hwmon uses fanX_target for fan speed control. It expects
the fan driver to have `fanX_target` sysfs attribute, e.g. Witherspoon's
max31875 driver.
For the fans with `fanX_target`, phosphor-hwmon creates "Target" property on
the fan sensor Dbus object;
Then phosphor-fan-presence/control sets the "Target" property to do the
thermal control.

Other systems like Romulus do not have this attribute, and it uses `pwmX` to
control the fan speed instead. This is not supported in phosphor-hwmon yet.

To support such case, the proposal is to let phosphor-hwmon to create "Target"
property for the fans controlled by pwmX as well, via hwmon config file.

E.g. for Romulus, fan9, fan11, fan13 are used, where fan9 and fan13 are
controlled by pwm1, and fan11 is controlled by pwm2, then the config looks
like:
```
TARGET_fan9 = "pwm1"
TARGET_fan11 = "pwm2"
```

* When phosphor-hwmon sees TARGET_fanX, it creates "Target" property for this
fan, and use the related pwmX to control the fan speed;
* For the fan control yaml configs, define the relted fan zone and control
logic, where the fan
* phosphor-fan-presence/control uses the same "Target" property to set the
fan speed target, only that the value is range from 0~255.

The phosphor-hwmon changes are submitted at
https://gerrit.openbmc-project.xyz/#/c/8353/

Thanks

--
BRs,
Lei YU