[media] Add common video interfaces OF bindings documentation

This patch adds a document describing common OF bindings for video
capture, output and video processing devices. It is curently mainly
focused on video capture devices, with data busses defined by
standards such as ITU-R BT.656 or MIPI-CSI2.
It also documents a method of describing data links between devices.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>