fbdev: Add FOURCC-based format configuration API

This API will be used to support YUV frame buffer formats in a standard
way.

Last but not least, create a much needed fbdev API documentation and
document the format setting APIs.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>