regulator: Document PM2250 smd-rpm regulators

Document compatible for PM2250 smd-rpm regulators and list all of them.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Link: https://lore.kernel.org/r/20210926084549.29880-2-shawn.guo@linaro.org


Signed-off-by: Mark Brown <broonie@kernel.org>