ethdev: add PFCP header to flow API

ethdev: add PFCP header to flow API

This patch adds the new flow item RTE_FLOW_ITEM_TYPE_PFCP to flow API to
match a PFCP header.
Add sample PFCP rules for testpmd guide. Since Session Endpoint
Identifier (SEID) only will be present in PFCP Session header and PFCP
Session headers shall be identified when the S field is equal to 1, when
create rules for PFCP Session header with certain SEID the S field need
be set 1.

Signed-off-by: Xiao Zhang <xiao.zhang@intel.com>
Acked-by: Ori Kam <orika@mellanox.com>