regulator: act8945a: add regulator driver for ACT8945A

This patch adds new regulator driver to support ACT8945A MFD
chip's regulators.

The ACT8945A has three step-down DC/DC converters and four
low-dropout regulators.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>