nfp: flower: support ipv6 tunnel keep-alive messages from fw

FW sends an update of IPv6 tunnels that are active in a given period. Use
this information to update the kernel table so that neighbour entries do
not time out when active on the NIC.

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