ARM: mvebu: ensure the mdio node has a clock reference on Armada 370/XP

The mvmdio driver accesses some register of the Ethernet unit. It
therefore takes a reference and enables a clock. However, on Armada
370/XP, no clock specification was given in the Device Tree, which
leads the mvmdio driver to fail when being used as a module and loaded
before the mvneta driver: it tries to access a register from a
hardware unit that isn't clocked.

Cc: stable@vger.kernel.org
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1395790439-21332-2-git-send-email-thomas.petazzoni@free-electrons.com
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>