net: dsa: qca8k: extend slave-bus implementations

This patch implements accessors for the QCA8337 MDIO access
through the MDIO_MASTER register, which makes it possible to
access the PHYs on slave-bus through the switch. In cases
where the switch ports are already mapped via external
"phy-phandles", the internal mdio-bus is disabled in order to
prevent a duplicated discovery and enumeration of the same
PHYs. Don't use mixed external and internal mdio-bus
configurations, as this is not supported by the hardware.

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