arm: mvebu: remove 'clock-frequency' properties from Armada 370/XP Ethernet nodes

The mvneta driver for the Marvell Armada 370/XP Ethernet devices has
gained proper clock framework integration, and the corresponding
Device Tree nodes now have a correct 'clocks' pointer.

The 'clock-frequency' properties in the various .dts files for Armada
370/XP boards have therefore become useless.

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