Add support for PD PoE negotiation.

Add support for PD PoE negotiation.

Power requests refer to the power at the PSE.
Thus the loss offset caused by the cable has to be added
to the power request. Also the power received from the PSE
must subtract the cable loss to be compatible with lldp.
There are three TLVs for CDPv2 PoE negotiation.
Power Consumption: Current maximum power consumption of PD.
Power Request: Wanted maximum power consumption of PD.
Power Available: Power output from PSE.

Only used if lldp PoE is not supported by switch.

A cisco switch which does support both lldp and cdp will
use the protocol which is first to transmit a package.

Change-Id: Ib45ae582799c50a8d839120c28a5080e582c66e9