soc: qcom: smd: Represent smd edges as devices

By representing each edge as its own device the channels are no longer
tied to being parented by the same smd device and as such an edge can
live as children of e.g. remoteproc instances.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>