V4L2 clock and asynchronous probing APIs

As of 3.11-rc1 V4L2 clock and asynchronous probing APIs are in the mainline. This makes it possible to initialise the V4L2 subsystem from the Device Tree without having to impose artificial requirements on drivers' probing order. The V4L2 clock API also moves us closer to supporting the Common Clock Framework universally under V4L2, while also allowing platforms, not yet implementing CCF to use the same API. This also makes subdevice driver re-use between soc-camera and the rest of V4L2 easier.