eth: fbnic: Add support to fetch group stats

Add support for group stats for mac. The fbnic_set_counter help preserve
the default values for counters which are not touched by the driver.

The 'reset' flag in 'get_eth_mac_stats' allows to choose between
resetting the counter to recent most value or fetching the aggregated
values of the counter.

The 'fbnic_stat_rd64' read 64b stats counters in an atomic fashion using
read-read-read approach. This allows to isolate cases where counter is
moving too fast making accuracy of the counter questionable.

Command: ethtool -S eth0 --groups eth-mac
Example Output:
eth-mac-FramesTransmittedOK: 421644
eth-mac-FramesReceivedOK: 3849708
eth-mac-FrameCheckSequenceErrors: 0
eth-mac-AlignmentErrors: 0
eth-mac-OctetsTransmittedOK: 64799060
eth-mac-FramesLostDueToIntMACXmitError: 0
eth-mac-OctetsReceivedOK: 5134513531
eth-mac-FramesLostDueToIntMACRcvError: 0
eth-mac-MulticastFramesXmittedOK: 568
eth-mac-BroadcastFramesXmittedOK: 454
eth-mac-MulticastFramesReceivedOK: 276106
eth-mac-BroadcastFramesReceivedOK: 26119
eth-mac-FrameTooLongErrors: 0

Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>