media: v4l2-fwnode: add endpoint id field to v4l2_fwnode_link

A link is between two endpoints not between two ports to be more
precise. Add the local_id/remote_id field which stores the endpoint
reg/port property to the link. Now the link holds all necessary
information about a link.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>