regulator: act8865: prepare support for other act88xx devices

This patch prepares support for other devices in the act88xx family of
PMUs manufactured by Active-Semi.

http://www.active-semi.com/products/power-management-units/act88xx/

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Tested-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>