net/mlx5: add flex item operations

net/mlx5: add flex item operations

This patch is a preparation step of implementing
flex item feature in driver and it provides:

  - external entry point routines for flex item
    creation/deletion

  - flex item objects management over the ports.

The flex item object keeps information about
the item created over the port - reference counter
to track whether item is in use by some active
flows and the pointer to underlying shared DevX
object, providing all the data needed to translate
the flow flex pattern into matcher fields according
hardware configuration.

There is not too many flex items supposed to be
created on the port, the design is optimized
rather for flow insertion rate than memory savings.

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>