igc: Add support for CBS offloading

Implement support for Credit-based shaper(CBS) Qdisc hardware
offload mode in the driver. There are two sets of IEEE802.1Qav
(CBS) HW logic in i225 controller and this patch supports
enabling them in the top two priority TX queues.

Driver implemented as recommended by Foxville External
Architecture Specification v0.993. Idleslope and Hi-credit are
the CBS tunable parameters for i225 NIC, programmed in TQAVCC
and TQAVHC registers respectively.

In-order for IEEE802.1Qav (CBS) algorithm to work as intended
and provide BW reservation CBS should be enabled in highest
priority queue first. If we enable CBS on any of low priority
queues, the traffic in high priority queue does not allow low
priority queue to be selected for transmission and bandwidth
reservation is not guaranteed.

Signed-off-by: Aravindhan Gunasekaran <aravindhan.gunasekaran@intel.com>
Signed-off-by: Mallikarjuna Chilakala <mallikarjuna.chilakala@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>