arm: mvebu: add Ethernet controllers using mvneta driver for Armada 370/XP

The Armada 370 SoC has two network units, while the Armada XP has four
network units. The first two network units are common to both the
Armada XP and Armada 370, so they are added to armada-370-xp.dtsi,
while the other two network units are specific to the Armada XP and
therefore added to armada-xp.dtsi.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>