linux-imx

[media] v4l2-device.rst: add contents from v4l2-framework

Part of the contents of v4l2-framework is related to the
kAPI defined by v4l2-device. Move such contents to the
v4l2-device.rst.

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