[media] v4l2-subdev.rst: add documentation from v4l2-framework.rst

There are lots of documentation about V4L2 subdevices at
v4l2-framework.rst. Move them to its specific chapter at
v4l2-subdev.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>