net/mlx5: support reading clock

net/mlx5: support reading clock

Implements support for read_clock for the mlx5 driver. mlx5 supports
hardware timestamp offload, setting packets timestamp field to the
device clock. rte_eth_read_clock allows to read the device's current
clock value and therefore compare values on similar time base.

See rxtx_callbacks for an example.

Signed-off-by: Tom Barbette <barbette@kth.se>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>