net: mscc: ocelot: add support for all sorts of standardized counters present in DSA

DSA is integrated with the new standardized ethtool -S --groups option,
but the felix driver only exports unstructured statistics.

Reuse the array of 64-bit statistics collected by ocelot_check_stats_work(),
but just export select values from it.

Since ocelot_check_stats_work() runs periodically to avoid 32-bit
overflow, and the ethtool calling context is sleepable, we update the
64-bit stats one more time, to provide up-to-date values. The locking
scheme with a mutex followed by a spinlock is a bit hard to digest, so
we create and use a ocelot_port_stats_run() helper with a callback that
populates the ethool stats group the caller is interested in.

The exported stats are:
ethtool -S swp0 --groups eth-phy
ethtool -S swp0 --groups eth-mac
ethtool -S swp0 --groups eth-ctrl
ethtool -S swp0 --groups rmon
ethtool --include-statistics --show-pause swp0

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>