net/mlx5: support flow tag

net/mlx5: support flow tag

Add support of new rte_flow item and action - TAG and SET_TAG. TAG is
a transient value which can be kept during flow matching.

This is supported through device metadata register reg_c[]. Although
there are 8 registers are available on the current mlx5 device,
some of them can be reserved for firmware or kernel purposes.
The availability should be queried by iterative trial-and-error
mlx5_flow_discover_mreg_c() routine.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>