soc: qcom: Add device tree binding for GLINK RPM

Add device tree binding documentation for the Qualcomm GLINK RPM, used
for communication with the Resource Power Management subsystem in
various Qualcomm SoCs.

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