nfp: link basic ethtool ops to representors

Start linking ethtool ops to representors.  Begin by adding
a separate ops structure and providing link state.  Next
patches will convert appropriate functions to only use nfp_port,
which will make them reusable on representors.

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>