net: dsa: introduce dsa_is_upstream_port and dsa_switch_is_upstream_of

In preparation for the new cross-chip notifiers for host addresses,
let's introduce some more topology helpers which we are going to use to
discern switches that are in our path towards the dedicated CPU port
from switches that aren't.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>