regulator: tps65910: Provide settling time for enabling rails

There is settling time for each rails when it is switched to
ON. Implementing enable time for returning proper settling time
of regulator rails when it is enabled.
Filling the on-time for each rail as per tps65910/tps65911
datasheets.

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