Documentation: admin-guide: media: add rockchip camera interface

Documentation: admin-guide: media: add rockchip camera interface

Add a document that describes the different variants of the Rockchip
Camera Interface (CIF), their hardware layout, as well as their
representation in the media controller centric rkcif device driver,
which is located under drivers/media/platform/rockchip/rkcif.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Michael Riesch <michael.riesch@collabora.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>