ARM: Kirkwood: Fix missing clk for USB device.

Without the clock being held by a driver, it gets turned off at a bad
time causing the SoC to lockup. This is often during reboot.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Stefan Peter <s.peter@mpl.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>