regulator: tps65910: Sleep control through external inputs

Add support for sleep controls of different regulator through
external inputs EN1, EN2 or EN3.
Each regulator's output will be active when its external
input is high and turns to OFF/Low power mode when its
external input is low.
The configuration parameters for sleep control is provided through
board specific platform data.

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