regulator: palmas: support for external regulator through control outputs

Palmas device have control outputs like REGEN1, REGEN2, REGEN3,
SYSEN1 and SYSEN2. These control outputs can be used for controlling
external voltage switches to enabled/disable voltage outputs.

Add support of these control outputs through regulator framework.

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