net/sfc: support SW stats groups

net/sfc: support SW stats groups

Add support for grouping SW stats together. When stats are
grouped the corresponding stats values for each queue
are obtained during calling one read callback. This is useful
to group per-queue stats 'packets' and 'bytes' to keep stats
consistent, i.e. a number of bytes corresponds to a number of
packets. These stats will be added in the following patches.

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>