nfp: flower: turn on recirc and merge hint support in firmware

Write to a FW symbol to indicate that the driver supports flow merging. If
this symbol does not exist then flow merging and recirculation is not
supported on the FW. If support is available, add a stub to deal with FW
to kernel merge hint messages.

Full flow merging requires the firmware to support of flow mods. If it
does not, then do not attempt to 'turn on' flow merging.

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>