Documentation/devicetree/bindings/net/dsa/dsa.txt says that the phy-mode
property should be specified on port nodes. However, the microchip
drivers read it from the switch node.

Let the driver use the per-port property and fall back to the old
location with a warning.

Fix in-tree users.
Signed-off-by: Helmut Grohne <helmut.grohne@intenta.de>
Link: https://lore.kernel.org/netdev/20200617082235.GA1523@laureti-dev/Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>