regulator: DT: Add settling time property for non-linear voltage change

Some regulators (some PWM regulators) have the voltage transition
exponentially. On such cases, the settling time for voltage change
is treated as constant time.

Add DT property for providing the settling time for any level of
voltage change for non-linear voltage change.

signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>

Signed-off-by: Mark Brown <broonie@kernel.org>