media: imx-csi: Input connections to CSI should be optional

Some imx platforms do not have fwnode connections to all CSI input
ports, and should not be treated as an error. This includes the
imx6q SabreAuto, which has no connections to ipu1_csi1 and ipu2_csi0.
Return -ENOTCONN in imx_csi_parse_endpoint() so that v4l2-fwnode
endpoint parsing will not treat an unconnected CSI input port as
an error.

Fixes: c893500a ("media: imx: csi: Register a subdev notifier")

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Cc: stable@vger.kernel.org
Tested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>