net/mlx5: E-Switch, Introduce global tables

Currently, flow tables are automatically connected according to their
<chain,prio,level> tuple.

Introduce global tables which are flow tables that are detached from the
eswitch chains processing, and will be connected by explicitly referencing
them from multiple chains.

Add this new table type, and allow connecting them by refenece.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>