[media] uvcvideo: Add driver documentation

This adds detailed documentation about the driver's extension unit control
features. Part of it has been adopted from the dynctrl.txt that used to be
available in the driver's previous Subversion repository.

The documentation has been rewritten to match the current implementation, in
particular to capture the new XU control support including UVCIOC_CTRL_QUERY.

Signed-off-by: Martin Rubli <martin_rubli@logitech.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>