Add dt-bindings for sm6125 SoC RPM Power Domains

Signed-off-by: Martin Botka <martin.botka@somainline.org>
[bjorn: Added compatible to binding as well]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211130212332.25401-1-martin.botka@somainline.org