dt-bindings: power: rpmpd: Add MSM8939 RPM power domains

MSM8939 has three RPM power domains: VDDCX and VDDMX and VDDMDCX.
Add the device tree bindings to manage them through rpmpd.

Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Link: https://lore.kernel.org/r/20200930100145.9457-2-jun.nie@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>