examples/rxtx_callbacks: support HW timestamp

examples/rxtx_callbacks: support HW timestamp

Use rxtx callback to demonstrate a way to use rte_eth_read_clock to
convert the hardware timestamps to an amount of cycles.

This allows to get the amount of time the packet spent since its entry
in the device. While the regular latency only shows the latency from
when it entered the software stack.

Signed-off-by: Tom Barbette <barbette@kth.se>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>