dt-binding: remoteproc: wcnss: Allow describing smd edge

Allow the associated smd edge to be described within the wcnss
remoteproc node. This creates a bond between the remoteproc and the
associated smd channels and devices, showing the interaction between the
two parts and provides both a natural reference to the other.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>