event/sw: add new software-only eventdev driver

event/sw: add new software-only eventdev driver

This adds the minimal changes to allow a SW eventdev implementation to
be compiled, linked and created at run time. The eventdev does nothing,
but can be created via vdev on commandline, e.g.

  sudo ./x86_64-native-linuxapp-gcc/app/test --vdev=event_sw0
  ...
  PMD: Creating eventdev sw device event_sw0, numa_node=0, sched_quanta=128
  RTE>>

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