ARM: dts: vf610-zii-dev-rev-b: specify phy-mode for external PHYs

Specify the phy-mode for the external PHYs on the third switch on the
ZII development rev B board so phylink and phylib knows what mode these
interfaces are configured for.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>