dt-bindings: power: rpmpd: Add MSM8916 RPM power domains

MSM8916 has two RPM power domains: VDDCX and VDDMX.
Add the device tree bindings to manage them through rpmpd.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20200916104135.25085-3-stephan@gerhold.net


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