media: uapi: pixfmt-luma: Document MIPI CSI-2 packing

The Y10P, Y12P and Y14P format variants are packed according to
the RAW10, RAW12 and RAW14 formats as defined by the MIPI CSI-2
specification. Document it.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>