ethdev: add field for device data per process

ethdev: add field for device data per process

Primary and secondary processes share a per-device private data. With
current design it is not possible to have data per-device per-process.
This is required for handling properly the CPP interface inside the NFP
PMD with multiprocess support.

There is also at least another PMD driver, tap, with similar
requirements for per-process device data.

Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>