net: dsa: Support internal phy on 'cpu' port

This adds support for enabling the internal PHY for a 'cpu' port.
It has been tested on GE B850v3,  B650v3 and B450v3, which have a
built-in MV88E6240 switch hardwired to a PCIe based network card.
On these machines the internal PHY of the i210 network card and
the Marvell switch are connected to each other and must be enabled
for properly using the switch. While the i210 PHY will be enabled
when the network interface is enabled, the switch's port is not
exposed as network interface. Additionally the mv88e6xxx driver
resets the chip during probe, so the PHY is disabled without this
patch.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>