nfp: add app pointer to port representors

nfp_apps can currently associate their structures with vNICs but
not representors.  Add app priv pointer to representors as well.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>