arm: mvebu: Armada XP MV78230 has only three Ethernet interfaces

We originally thought that the MV78230 variant of the Armada XP had
four Ethernet interfaces, like the other variants MV78260 and
MV78460. In fact, this is not true, and the MV78230 has only three
Ethernet interfaces.

So, the definitions of the Ethernet interfaces is now done as follows:

 * armada-370-xp.dtsi: definitions of the first two interfaces, that
   are common to Armada 370 and Armada XP

 * armada-xp.dtsi: definition of the third interface, common to all
   Armada XP variants.

 * armada-xp-mv78260.dtsi and armada-xp-mv78460.dtsi: definition of
   the fourth interface.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>