Multi-channel clients split between several drivers need a way to close
individual channels, as these drivers might be removed individually.
With this in place the responsibility of closing additionally opened
channels to the client as well only concerning smd about the primary
channel.

With this approach we will only trigger removal of SMD devices based on
the state of the primary channel, however we get in sync with how rpmsg
works.
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: NAndy Gross <andy.gross@linaro.org>