nfp: app callbacks for SRIOV

Add app-callbacks for app-specific initialisation of SRIOV.

Disabling SRIOV is brought forward in nfp_pci_remove()
so that nfp_app_sriov_disable is called while the app still exists.

This is intended to be used to implement representor netdevs for virtual
ports.

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>