The ocelot counters are 32-bit and require periodic reading, every 2
seconds, by ocelot_port_update_stats(), so that wraparounds are
detected.

Currently, the counters reported by ocelot_get_stats64() come from the
32-bit hardware counters directly, rather than from the 64-bit
accumulated ocelot->stats, and this is a problem for their integrity.

The strategy is to make ocelot_get_stats64() able to cherry-pick
individual stats from ocelot->stats the way in which it currently reads
them out from SYS_COUNT_* registers. But currently it can't, because
ocelot->stats is an opaque u64 array that's used only to feed data into
ethtool -S.

To solve that problem, we need to make ocelot->stats indexable, and
associate each element with an element of struct ocelot_stat_layout used
by ethtool -S.

This makes ocelot_stat_layout a fat (and possibly sparse) array, so we
need to change the way in which we access it. We no longer need
OCELOT_STAT_END as a sentinel, because we know the array's size
(OCELOT_NUM_STATS). We just need to skip the array elements that were
left unpopulated for the switch revision (ocelot, felix, seville).
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>