nfp: compile flower vxlan tunnel metadata match fields

Compile ovs-tc flower vxlan metadata match fields for offloading. Only
support offload of tunnel data when the VXLAN port specifically matches
well known port 4789.

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