eventdev: introduce event driven programming model

eventdev: introduce event driven programming model

In a polling model, lcores poll ethdev ports and associated
rx queues directly to look for packet. In an event driven model,
by contrast, lcores call the scheduler that selects packets for
them based on programmer-specified criteria. Eventdev library
adds support for event driven programming model, which offer
applications automatic multicore scaling, dynamic load balancing,
pipelining, packet ingress order maintenance and
synchronization services to simplify application packet processing.

By introducing event driven programming model, DPDK can support
both polling and event driven programming models for packet processing,
and applications are free to choose whatever model
(or combination of the two) that best suits their needs.

This patch adds the eventdev specification header file.

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>