regulator: DT: Add properties for asymmetric settling times

Some regulators have different settling times for voltage increases and
decreases. Add DT properties to define separate settling times for up-
and downward voltage changes.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/regulator.txt

@@ -24,6 +24,14 @@ Optional properties:
 - regulator-settling-time-us: Settling time, in microseconds, for voltage
   change if regulator have the constant time for any level voltage change.
   This is useful when regulator have exponential voltage change.
+- regulator-settling-time-up-us: Settling time, in microseconds, for voltage
+  increase if the regulator needs a constant time to settle after voltage
+  increases of any level. This is useful for regulators with exponential
+  voltage changes.
+- regulator-settling-time-down-us: Settling time, in microseconds, for voltage
+  decrease if the regulator needs a constant time to settle after voltage
+  decreases of any level. This is useful for regulators with exponential
+  voltage changes.
 - regulator-soft-start: Enable soft start so that voltage ramps slowly
 - regulator-state-mem sub-root node for Suspend-to-RAM mode
   : suspend to memory, the device goes to sleep, but all data stored in memory,