regulator: Document new regulator-max-step-microvolt property

Certain hardware may require supply voltage to be changed in steps. Define
new property that allow to describe such hardware.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>