net/sfc: support concept of switch domains/ports

net/sfc: support concept of switch domains/ports

A later patch will add support for RTE flow action PORT_ID
to MAE backend. The driver has to ensure that such actions
refer to RTE ethdev instances deployed on top of the same
physical device. Also, the driver needs a means to find
sibling RTE ethdev instances when parsing such actions.

In order to solve these problems, add a switch infrastructure
which allocates switch domains based on persistence of device
serial number string across switch ports included in a domain.
Explain mapping between RTE switch port IDs and MAE endpoints.

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