Document the "qcom,rpm-pm8909-regulators" compatible for describing
the regulators available in the PM8909 PMIC (controlled via the RPM
firmware).

PM8909 is very similar to the existing PM8916 but lacks 3 of the
regulators (s3, s4 and l16).
Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220623094614.1410180-3-stephan.gerhold@kernkonzept.comSigned-off-by: Mark Brown <broonie@kernel.org>