V4L/DVB (13651): sh_mobile_ceu_camera: document the scaling and cropping algorithm

The sh_mobile_ceu_camera driver implements an advanced algorithm, combining
scaling and cropping on the client and on the host. Due to its complexity the
algorithm deserves separate documentation.

 create mode 100644 Documentation/video4linux/sh_mobile_ceu_camera.txt

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