arm: mvebu: add 'clocks' property to Ethernet nodes for Armada 370/XP SoCs

The mvneta driver now understands a standard 'clocks' clock pointer
property in the Device Tree nodes for the Ethernet devices, so we add
the right clock reference for the different Ethernet ports of the
Armada 370/XP SoCs.

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