mlxsw: reg: Add support for counters on RATR

In order to add the ability for setting counters on nexthops the RATR
register should be extended.

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>