media: nxp: imx8-isi: Mark all crossbar sink pads as MUST_CONNECT

All the sink pads of the crossbar switch require an active link if
they're part of the pipeline. Mark them with the
MEDIA_PAD_FL_MUST_CONNECT flag to fail pipeline validation if they're
not connected. This allows removing a manual check when translating
streams.

Cc: stable@vger.kernel.org # 6.1
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>