mlxsw: reg: Add Router Interface Counter Register

The RICNT register retrieves per port performance counter. It will be
used to query the router interfaces statistics.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>