dm: regulator: add implementation of driver model regulator uclass

This commit introduces the implementation of dm regulator API.
Device tree support allows for auto binding. And by the basic
uclass operations, it allows to driving the devices in a common
way. For detailed informations, please look into the header file.

Core files:
- drivers/power/regulator-uclass.c - provides regulator common functions api
- include/power/regulator.h - define all structures required by the regulator

Changes:
- new uclass-id: UCLASS_REGULATOR
- new config: CONFIG_DM_REGULATOR

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>