common/sfc_efx/base: handle Rx events for Riverhead

common/sfc_efx/base: handle Rx events for Riverhead

Rx event on Riverhead provides a number of received packets and
no classification/offloads information is available Rx event.

Introduce a new event callback to be implemented by drivers.
The callback provides information about the number of completed
packets. libefx-based drivers should implement the new callback
for Riverhead and keep the old one for Siena and EF10 NICs.
The new callback may be used for Medford2 NO_CONT_EV Rx mode
support.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>