drivers: video: controls: document each control and move classes

Make controls more readable and intuitive to extend, with definition
of each control value to help disambiguate between similar controls,
such as BRIGHTNESS vs GAIN.

Move the class definition to the top of each relevant class, like it
is on Linux for the V4L2_CID_..._CLASS_BASE variables.

Signed-off-by: Josuah Demangeon <me@josuah.net>