mlxsw: spectrum: Add support for setting counters on nexthops

Add support for setting counters on nexthops based on dpipe's adjacency
table counter status. This patch also adds the ability for getting the
counter value, which will be used by the dpipe adjacency table dump
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>