dt-bindings: regulator: Describe Qualcomm REFGEN regulator

dt-bindings: regulator: Describe Qualcomm REFGEN regulator

Modern Qualcomm SoCs have a REFGEN (reference voltage generator)
regulator, providing reference voltage to on-chip IP, like PHYs.
It's controlled through MMIO and we can toggle it or read its state back.

Describe it.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230628-topic-refgen-v3-1-9fbf0e605d23@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>