media: v4l2-tpg: Add support for the new YUVA and YUVX formats

Extend the TPG to support generating the newly added YUVA and YUVX pixel
formats.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>