Implement IPMI POH functionality

[Ratan Gupta <ratagupt@linux.vnet.ibm.com>]

Hi,

I would be explaining a rough idea of how we plan to go about,
This is related to https://github.com/openbmc/openbmc/issues/2979

Please share your thoughts and feedback on this proposal.

=>we were planning to add the property in the chassis interface
   which tells that since how many hours chassis was powered on.

=>This property gets updated by internal timer which wakes up after
   an hour and updates this property if chassis is powered on.

    1) Should we set the uptime to 0 if chassis state has been 
transitioned from
    PowerOn-->PowerOff

                              or

    2) Don't update this property for the chassis power off duration and 
update
    this property when the chassis state changes to PowerOn.

    What is community view on this?

    Our opinion is that we should go with option 2 as other 
implementation(AMI)
    does the same.

=> In factory reset case this property will reset to 0.

=>If BMC has been rebooted due to any reason,This property will not
   get updated for that duration and when BMC comes back,This timer
   kicks off and updates this property and it would be cumulative
   on what was the earlier value before BMC reboot.

=> Chassis object would be saved in nonvolatile storage to keep the last 
POH counter value.

=> Get functionality would be allowed,set would not be allowed from IPMI 
interface.

Regards
Ratan Gupta