net/sfc: introduce flow allocation and free path

Riverhead boards maintain support for MAE, a low-level Match-Action Engine.
The upcoming patches will bring support for MAE to RTE flow implementation.

A follow-up patch will introduce MAE-specific specification cleanup method.
In order to prepare for the patch, introduce flow allocation and free path.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>