net: dsa: mv88e6xxx: Fix binding documentation for MDIO busses

The MDIO busses are switch properties and so should be inside the
switch node. Fix the examples in the binding document.

Reported-by: 尤晓杰 <yxj790222@163.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Fixes: a3c53be5 ("net: dsa: mv88e6xxx: Support multiple MDIO busses")
Signed-off-by: David S. Miller <davem@davemloft.net>