media: uapi: Clarify MBUS color component order for serial buses

media: uapi: Clarify MBUS color component order for serial buses

The subdev format documentation has a subsection describing how to use
the media bus pixel codes for serial buses. While it describes the
sampling part well, it doesn't really describe the current convention
used for the components order.

Let's improve that.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>