media: doc: pixfmt-packed-yuv: Fill padding bits with 'X'

The padding bits are left blank, which look weird in the XYUV format,
and, worse, may lead to the VUYX format to be understand as consuming 3
bytes per pixel. Add 'X' for padding bits as we do for RGB formats.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>