regulator: add a regulator driver for the AS3711 PMIC

This driver supports the 4 DCDC and 8 LDO regulators on the AS3711 PMIC.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>