regulator: as3722: add regulator driver for AMS AS3722

The AMS AS3722 is a compact system PMU suitable for mobile phones,
tablets etc. It has 4 DCDC step down regulators, 3 DCDC step down
controller, 11 LDOs.

Add a driver to support accessing the DCDC/LDOs found on the AMS
AS3722 PMIC using regulators.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Florian Lobmaier <florian.lobmaier@ams.com>
Signed-off-by: Mark Brown <broonie@linaro.org>