[media] Documentation: devicetree: Update Samsung FIMC DT binding

This patch documents following updates of the Exynos4 SoC camera subsystem
devicetree binding:

 - addition of #clock-cells and clock-output-names properties to 'camera'
   node - these are now needed so the image sensor sub-devices can reference
   clocks provided by the camera host interface,
 - dropped a note about required clock-frequency properties at the
   image sensor nodes; the sensor devices can now control their clock
   explicitly through the clk API and there is no need to require this
   property in the camera host interface binding.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>