rpmsg: char: Introduce the "rpmsg-raw" channel

For the rpmsg virtio backend, the current implementation of the rpmsg char
only allows to instantiate static(i.e. prefixed source and destination
addresses) end points, and only on the Linux user space initiative.

This patch defines the "rpmsg-raw" channel and registers it to the rpmsg bus.
This registration allows:
- To create the channel at the initiative of the remote processor
  relying on the name service announcement mechanism. In other words the
  /dev/rpmsgX interface is instantiate by the remote processor.
- To use the channel object instead of the endpoint, thus preventing the
  user space from having the knowledge of the remote processor's
  endpoint addresses.
- To rely on udev to be inform when a /dev/rpmsgX is created on remote
  processor request, indicating that the remote processor is ready to
  communicate.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220124102524.295783-11-arnaud.pouliquen@foss.st.com