net: dsa: qca8k: add support for internal phy and internal mdio

Add support to setup_mdio_bus for internal phy declaration. Introduce a
flag to use the legacy port phy mapping by default and use the direct
mapping if a mdio node is detected in the switch node. Register a
dedicated mdio internal mdio bus to address the different mapping
between port and phy if the mdio node is detected.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>