[media] v4l: Document raw bayer 4CC codes

Document guidelines how 4CC codes should be named. Only raw bayer is
included currently. Other formats should be documented later on.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>