dt-binding: soc: qcom: smd: Add label property

The label property can be used to specify a name of the edge, for
consistent naming purposes.

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