regulator: Introduce Qualcomm REFGEN regulator driver

Modern Qualcomm SoCs have a REFGEN (reference voltage generator)
regulator, providing reference voltage to on-chip IP, like PHYs.

Add a driver to support toggling that regulator.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20230628-topic-refgen-v3-2-9fbf0e605d23@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>