rpmsg: qcom_smd: Add support for "label" property

Add support for the "label" property, used to give the edge a name other
than the one of the DT node. This allows the implementor to provide
consistently named edges when using the rpmsg character device.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
(cherry picked from commit 5e53c42c)