linux-imx

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

There are two additional subdev-specific sections at the
v4l2-framework file. Move them to the subdev chapter, in order
to better organize the book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>