regulator: tps65910: Provide settling time for DCDC voltage change

Settling time is require when there is dcdc rail's voltage change.
Returning proper delay time for dcdc voltage change to settle down
the output voltage to new value.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>