media: v4l2-fwnode: add v4l2_fwnode_connector

Currently every driver needs to parse the connector endpoints by it self.
This is the initial work to make this generic. A generic connector has
common members and connector specific members. The common members are:
  - type
  - label (optional)
  - links
  - nr_of_links

The specific members are stored within a union, since only one of them
can be available at the time. Since this is the initial support the
patch adds only the analog-connector specific ones.

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>