ramips: mt7620n: add mdio node and disable port4 by default

ramips: mt7620n: add mdio node and disable port4 by default

Revert the changes I applied to aa5014dd1a58 ("ramips: mt7620n: enable
port 4 as EPHY by default").

The driver expects a node mdio-bus to be present, regardless of the
actual node status. If the node is missing the driver fails to load  with

  mtk_soc_eth 10100000.ethernet: no mdio-bus child node found

Disable port4 by default again. If the port is enabled but not present, a
"invalid port id 4" warning is shown during boot.

Fixes: FS#1428

Signed-off-by: Mathias Kresin <dev@kresin.me>