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.

Change-Id: I86510c81a8386e230cf4652079baf52b401076e8
Signed-off-by: Chris Lew <clew@codeaurora.org>