igc: add support to get frame preemption statistics via ethtool

Implemented "ethtool --include-statistics --show-mm" callback for IGC.

Tested preemption scenario to check preemption statistics:
1) Trigger verification handshake on both boards:
    $ sudo ethtool --set-mm enp1s0 pmac-enabled on
    $ sudo ethtool --set-mm enp1s0 tx-enabled on
    $ sudo ethtool --set-mm enp1s0 verify-enabled on
2) Set preemptible or express queue in taprio for tx board:
    $ sudo tc qdisc replace dev enp1s0 parent root handle 100 taprio \
      num_tc 4 map 3 2 1 0 3 3 3 3 3 3 3 3 3 3 3 3 \
      queues 1@0 1@1 1@2 1@3 base-time 0 sched-entry S F 100000 \
      fp E E P P
3) Send large size packets on preemptible queue
4) Send small size packets on express queue to preempt packets in
   preemptible queue
5) Show preemption statistics on the receiving board:
   $ ethtool --include-statistics --show-mm enp1s0
     MAC Merge layer state for enp1s0:
     pMAC enabled: on
     TX enabled: on
     TX active: on
     TX minimum fragment size: 64
     RX minimum fragment size: 60
     Verify enabled: on
     Verify time: 128
     Max verify time: 128
     Verification status: SUCCEEDED
     Statistics:
      MACMergeFrameAssErrorCount: 0
      MACMergeFrameSmdErrorCount: 0
      MACMergeFrameAssOkCount: 511
      MACMergeFragCountRx: 764
      MACMergeFragCountTx: 0
      MACMergeHoldCount: 0

Co-developed-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Co-developed-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>