nfp: introduce very minimal nfp_app

Introduce a concept of an application.  For now it's just grouping
pointers and serving as a layer of indirection.  It will help us
weaken the dependency on nfp_net in ethtool code.  Later series
will flesh out support for different apps in the driver.

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>