mlxsw: reg: Add Router Adjacency Table Activity Dump Register

The RATRAD register is used to dump and optionally clear activity bits
of router adjacency table entries. Will be used by the next patch to
query and clear the activity of nexthop buckets in a resilient nexthop
group.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>