dt-bindings: net: dsa: make phylink bindings required for CPU/DSA ports

It is desirable that new DSA drivers are written to expect that all
their ports register with phylink, and not rely on the DSA core's
workarounds to skip this process.

To that end, DSA is being changed to warn existing drivers when such DT
blobs are in use, and to opt new drivers out of the workarounds.

Introduce another layer of validation in the DSA DT schema, and assert
that CPU and DSA ports must have phylink-related properties present.

Suggested-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>