net: mvneta: add clk support

Now that the Armada 370/XP platform has gained proper integration with
the clock framework, we add clk support in the Marvell Armada 370/XP
Ethernet driver.

Since the existing Device Tree binding that exposes a
'clock-frequency' property has never been exposed in any stable kernel
release, we take the freedom of removing this property to replace it
with the standard 'clocks' clock pointer property.

The Device Tree binding documentation is updated accordingly.

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