mlxsw: core_acl_flex_actions: Add IGNORE_ACTION

Add the IGNORE_ACTION which is used to ignore basic switching functions
such as learning on a per-packet basis.

The action will be prepended to the FORWARDING_ACTION in subsequent
patches.

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