media: v4l: Add packed YUV 4:4:4 YUVA and YUVX pixel formats

The new YUVA and YUVX are permutations of the existing AYUV and XYUV
formats. They are use by the NXP i.MX8 ISI hardware.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.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>