devicetree: bindings: dsa: qca8k: Document internal mdio definition

Document new way of declare mapping of internal PHY to port.
The new implementation directly declare the PHY connected to the port
by adding a node in the switch node. The driver detect this and register
an internal mdiobus using the mapping defined in the mdio node.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>