V4L2 clocks
  -----------
  
  .. attention::
  
  	This is a temporary API and it shall be replaced by the generic
  	clock API, when the latter becomes widely available.
  
  Many subdevices, like camera sensors, TV decoders and encoders, need a clock
  signal to be supplied by the system. Often this clock is supplied by the
  respective bridge device. The Linux kernel provides a Common Clock Framework for
  this purpose. However, it is not (yet) available on all architectures. Besides,
  the nature of the multi-functional (clock, data + synchronisation, I2C control)
  connection of subdevices to the system might impose special requirements on the
  clock API usage. E.g. V4L2 has to support clock provider driver unregistration
  while a subdevice driver is holding a reference to the clock. For these reasons
  a V4L2 clock helper API has been developed and is provided to bridge and
  subdevice drivers.
  
  The API consists of two parts: two functions to register and unregister a V4L2
  clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control
  a clock object, similar to the respective generic clock API calls:
  v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(),
  v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide
  clock operations that will be called when clock users invoke respective API
  methods.
  
  It is expected that once the CCF becomes available on all relevant
  architectures this API will be removed.