net: mvpp2: debugfs: Allow reading the flow table from debugfs

The Classifier flow table is the central part of the PPv2 Classifier,
since it describes all classification steps performed for each flow.

It has 512 entries, shared between all ports, which are divided into
sequences that are pointed-to by the decoding table. Being able to see
which entries in the flow table were hit is a key point when
implementing and debugging classification offload.

This commit allows reading each flow table entry's hit count
independently, with a clear-on-read behaviour.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>