mlxsw: spectrum: Add support for TC flower offload statistics

Add support for TC flower offload statistics including number of packets,
bytes and last use timestamp. Currently the statistics are gathered on a
per-rule basis.

Signed-off-by: Arkadi Sharshvesky <arkadis@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>