app/testpmd: add Tx scheduling command

app/testpmd: add Tx scheduling command

This commit adds testpmd capability to provide timestamps on the packets
being sent in the txonly mode. This includes:

 - SEND_ON_TIMESTAMP support
   new device Tx offload capability support added, example:

     testpmd> port config 0 tx_offload send_on_timestamp on

 - set txtimes, registers field and flag, example:

     testpmd> set txtimes 1000000,0

   This command enables the packet send scheduling on timestamps if
   the first parameter is not zero, generic format:

     testpmd> set txtimes (inter),(intra)

   where:

     inter - is the delay between the bursts in the device clock units.
     If "intra" (next parameter) is zero, this is the time between the
     beginnings of the first packets in the neighbour bursts, if "intra"
     is not zero, "inter" specifies the time between the beginning of
     the first packet of the current burst and the beginning of the last
     packet of the previous burst. If "inter"parameter is zero the send
     scheduling on timestamps is disabled (default).

     intra - is the delay between the packets within the burst specified
     in the device clock units. The number of packets in the burst is
     defined by regular burst setting. If "intra" parameter is zero no
     timestamps provided in the packets excepting  the first one in the
     burst.

     As the result the bursts of packet will be transmitted with
     specific delay between the packets within the burst and specific
     delay between the bursts. The rte_eth_read_clock() is supposed to
     be engaged to get the current device clock value and provide the
     reference for the timestamps. If there is no supported
     rte_eth_read_clock() there will be no provided send scheduling on
     the device.

 - show txtimes, displays the timing settings
 - txonly burst time pattern

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>