ARM: mvebu: use the fixed-link PHY DT binding for the Armada XP Matrix board

The Armada XP Matrix board has an Ethernet PHY that isn't configurable
through the MDIO bus, so we use the newly introduced fixed-link PHY DT
binding to represent the PHY of this platform and get network working.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>