event/dlb2: add private data structures and constants

event/dlb2: add private data structures and constants

The header file dlb2_priv.h is used internally by the PMD.
It include constants, macros for device resources,
structure definitions for hardware interfaces and
software state, and various forward-declarations.
The header file rte_pmd_dlb2.h will be exported in a
subsequent patch, but is included here due to a data
structure dependency.

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