Add target interface for fan monitor

Current fan monitor assumes the use of the FanSpeed interface for fan
targets.
For fans controlled by pwm, FanPwm interface is added.

This commit adds a "target_interface" config parameter, so that user
can specify the interface for the fan targets.

E.g.

  - name: fan0
    has_target: true
    target_interface: xyz.openbmc_project.Control.FanPwm

This config is optional and defaults to FanSpeed, so the current code
will not be affected.

Tested: Use this config on Romulus, ensures fan monitor gets fan
        target from FanPwm interface and works OK.

Change-Id: I262a486c335b2b43a46af7abdd0e71e95a133b98
Signed-off-by: Lei YU <mine260309@gmail.com>
6 files changed