regulator: palmas: configure enable time for LDOs

As per datasheet (Referred TPS65913), the on-time for LDO is
500micro second. If LDO6 is in vibrator mode then the on-time
is 2000us.

Set the enable_time on regulator descriptor accordingly.

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