RDMA/mlx5: Attach a DEVX counter via raw flow creation

Allow a user to attach a DEVX counter via mlx5 raw flow creation. In order
to attach a counter we introduce a new attribute:

MLX5_IB_ATTR_CREATE_FLOW_ARR_COUNTERS_DEVX

A counter can be attached to multiple flow steering rules.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>