arm: dt: vf610-zii-devel-b: Make use of new DSA binding

Hang the three switches of the three MDIO busses using the new DSA
binding. Also, make use of the mdio-bus and explicitly list the phys
on one device. This is not required, but good for testing.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>