One helpful feature to help debug the Header Parser TCAM filter in PPv2
is to be able to see if the entries did match something when a packet
comes in. This can be done by using the built-in hit counter for TCAM
entries.

This commit implements reading the counter, and exposing its value on
debugfs for each filter entry.

The counter is a 16-bits clear-on-read value, located at:
 .../mvpp2/<controller>/parser/XXX/hits
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>