media: v4l: Add V4L2-PIX-FMT-Y12P format

This is a packed grey-scale image format with a depth of 12 bits per
pixel. Two consecutive pixels are packed into 3 bytes. The first 2 bytes
contain the 8 high order bits of the pixels, and the 3rd byte contains
the 4 least significants bits of each pixel, in the same order.

Add the entry in userspace API, and document it.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>