app/testpmd: support GTP PSC option in raw sets

app/testpmd: support GTP PSC option in raw sets

This patch add support for generating GTP PDU container
session option for the raw encap and raw decap sets.
The generated options is single 32-bit word with
minimal length specified as 4, no extra fields in the
option are supported. The option item must be preceded
with the GTP item itself, and GTP item flags must be
set accordingly:

  - E flag must be set, we are going to provide extension
  - S flag must be reset, because GTP item does not
    provide the value for SQN field, we can't fill this one
  - PN flag must be reset, no N-PDU value provided by
    GTP item either

The raw set example:

  set raw_encap 2 eth / ipv4 / udp /
                  gtp v_pt_rsv_flags is 0x34 / gtp_psc / end_set

Please, note - value 0x34 provides the required flag combination.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Ori Kam <orika@nvidia.com>