ARM: Kirkwood: Fix clk problems modular ethernet driver

When the ethernet driver was built as a module, it would lock the
machine when loaded. At boot the ethernet clks are unused, so get
turned off.  Later, when the module is loaded, the probe function
would access the hardware before the clock was restarted, and the
machine would lock. It has also been determined that when the clk is
turned off, the interface forgets its MAC address, which for most
systems, is set by the boot loader.

When the machine setup file creates a platform device for the
interface, prepare and enable the clock for the interface. This will
ensure it is not turned off. However, if the setup file only
instantiates one platform device, the other will have its clk
disabled, thus maybe saving a little power.

Report-by: Simon Baatz <gmbnomis@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Simon Baatz <gmbnomis@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>