nfp: general representor implementation

Provide infrastructure to create and destroy representors of a given type.

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>