dt-bindings: soc: qcom: Add label for GLINK bindings

There are GLINK clients who open the same channel on multiple GLINK
links. These clients need a way to distinguish which remoteproc they
are communicating to. Add a label property to identify the edge this
node represents.

Reviewed-by: Rob Herring <robh@kernel.org>
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>