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>

Documentation/devicetree/bindings/regulator/regulator.txt

@@ -79,6 +79,8 @@ Optional properties:
 - regulator-coupled-max-spread: Array of maximum spread between voltages of
   coupled regulators in microvolts, each value in the array relates to the
   corresponding couple specified by the regulator-coupled-with property.
+- regulator-max-step-microvolt: Maximum difference between current and target
+  voltages that can be changed safely in a single step.
 
 Deprecated properties:
 - regulator-compatible: If a regulator chip contains multiple