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>