net/sfc: support flow action count in transfer rules

net/sfc: support flow action count in transfer rules

For now, a rule may have only one dedicated counter, shared counters
are not supported.

HW delivers (or "streams") counter readings using special packets.
The driver creates a dedicated Rx queue to receive such packets
and requests that HW start "streaming" the readings to it.

The counter queue is polled periodically, and the first available
service core is used for that. Hence, the user has to specify at least
one service core for counters to work. Such a core is shared by all
MAE-capable devices managed by sfc driver.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ivan Malov <ivan.malov@oktetlabs.ru>