eventdev: add ring structure for events

eventdev: add ring structure for events

Add in a new rte_event_ring structure type and functions to allow events to
be passed core to core. This is needed because the standard rte_ring type
only works on pointers, while for events, we want to copy the entire, 16B
events themselves - not just pointers to them. The code makes extensive use
of the functions already defined in rte_ring.h

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>