nfp: add stats and xmit helpers for representors

Provide helpers for stats and xmit on representor netdevs.

Parts based on work by Bert van Leeuwen, Benjamin LaHaise and
Jakub Kicinski.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>