mlxsw: spectrum_router: Add support for setting counters on neighbors

Add support for setting counters on neighbors based on dpipe's host table
counter status. This patch also adds the ability for getting the counter
value, which will be used by the dpipe host table implementation in the
next patches.

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>