rpmsg: qcom_smd: add of_node node to edge device

This patch assigns the device node to the edge device, so that the edge
device drivers could read required device tree properties.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>