ARM: dts: armada-388-clearfog: remove duplicate mdio entry

The clearfog DTS should not be defining the on-board phy, this device
is located on the microsom.  Remove the duplicated definition.

Reported-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>