nfp: add devlink support

nfp: add devlink support

Add initial devlink support.  This patch simply switches allocation
of per-adapter structure to devlink's priv and register devlink
with empty ops table.  See following patches for implementation
of particular ops.

We should now clear the app pointer on exit, this is how devlink
callbacks will know app is not initialized.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>