rpmsg: glink: Store edge name for glink device

Channels may need to identify the edge their channel was probed for.
Store the edge name by reading the label property from device tree or
default to the node name.

Signed-off-by: Chris Lew <clew@codeaurora.org>
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>