media: v4l: async: Allow multiple connections between entities
When the v4l2-async framework was introduced, the use case for it was to
connect a camera sensor with a parallel receiver. Both tended to be rather
simple devices with a single connection between them.
The framework has been since improved in multiple ways but there are
limitations that have remained, for instance the assumption an async
sub-device is connected towards a single notifier and via a single link
only.
This patch enables connecting a sub-device to one or more notifiers
simultaneously, with one or more connections per notifier. The notifier
information is moved from the sub-device to the connection and the
connections in sub-device are no longer a pointer but a linked list.