mlxsw: spectrum: Add support for generic flow counter allocation

Add support for allocating generic flow counter. Generic flow counter
can count packets or packets and bytes and can be assigned to different
hardware processes. First use will be for counting packets and bytes of
ACL rules, and will be introduced in the following patches.

Signed-off-by: Arkadi Sharshevsky <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>