event/dlb2: add flexible interface

event/dlb2: add flexible interface

This commit introduces the flexible interface. This
interface allows the core code to operate in PF mode (direct
hardware access) or bifurcated mode (hardware configured via
kernel driver). This driver currently only supports PF mode
but bifurcated mode will be added in a future DPDK patch-set.
Note that the flexible interface is not used for data path
operations, and thus there are no performance concerns
related to the use of function pointers.

Signed-off-by: Timothy McDaniel <timothy.mcdaniel@intel.com>
Reviewed-by: Gage Eads <gage.eads@intel.com>