nfp: flower: support multiple memory units for filter offloads

Adds support for multiple memory units which are used for filter
offloads. Each filter is assigned a stats id, the MSBs of the id are
used to determine which memory unit the filter should be offloaded
to. The number of available memory units that could be used for filter
offload is obtained from HW. A simple round robin technique is used to
allocate and distribute the ids across memory units.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>