nfp: add support for control messages for flower app

In preparation for adding a new flower app - targeted at offloading
the flower classifier - provide support for control message that it will
use to communicate with the NFP.

Based in part on work by Bert van Leeuwen.

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>