dt-bindings: net: dsa: qca8k: utilize shared dsa.yaml

The dsa.yaml binding contains duplicated bindings for address and size
cells, as well as the reference to dsa-port.yaml. Instead of duplicating
this information, remove the reference to dsa-port.yaml and include the
full reference to dsa.yaml.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>