mfd: Add support for ams AS3722 PMIC

The ams AS3722 is a compact system PMU suitable for mobile phones,
tablets etc. It has 4 DC/DC step-down regulators, 3 DC/DC step-down
controller, 11 LDOs, RTC, automatic battery, temperature and
over-current monitoring, 8 GPIOs, ADC and a watchdog.

Add MFD core driver for the AS3722 to support core functionality.

Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Florian Lobmaier <florian.lobmaier@ams.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>