net/avp: add public header files

net/avp: add public header files

Adds public/exported header files for the AVP PMD.  The AVP device is a
shared memory based device.  The structures and constants that define the
method of operation of the device must be visible by both the PMD and the
host DPDK application.  They must not change without proper version
controls and updates to both the hypervisor DPDK application and the PMD.

The hypervisor DPDK application is a Wind River Systems proprietary
virtual switch.

Signed-off-by: Allain Legacy <allain.legacy@windriver.com>
Signed-off-by: Matt Peters <matt.peters@windriver.com>
Acked-by: Vincent Jardin <vincent.jardin@6wind.com>