nfp: store pointer to MAC statistics in nfp_port

Store pointer to device memory containing MAC statistics
in nfp_port.  This simplifies representor code and will
be used to dump those statistics in ethtool as well.

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