net/mlx5: introduce hardware steering enable routine

net/mlx5: introduce hardware steering enable routine

The new hardware steering engine relies on using dedicated steering WQEs
instead of writing to the low-level steering table entries directly.
In the first implementation the hardware steering engine supports the
new queue based Flow API, the existing synchronous non-queue based Flow
API is not supported.

A new dv_flow_en value 2 is added to manage mlx5 PMD steering engine:

dv_flow_en	rte_flow API	rte_flow_async API
------------------------------------------------
 0		support		not support
 1		support		not support
 2		not support	support

This commit introduces the extra dv_flow_en = 2 to specify the new
flow initialize and manage operation routine.

Signed-off-by: Suanming Mou <suanmingm@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>