regulator: helper: Add helper to configure active-discharge using regmap

Add helper function to set the state of active-discharge of
regulator using regmap. The HW regulator driver can directly
use this by providing the necessary information in the regulator
descriptor.

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