dt-bindings: net: dsa: qca8k: support internal mdio-bus

This patch updates the qca8k's binding to document to the
approach for using the internal mdio-bus of the supported
qca8k switches.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>