dm: regulator: add max77686 regulator driver

This commit adds support to MAX77686 regulator driver,
based on a driver model regulator's API. It implements
almost all regulator operations, beside those for setting
and geting the Current value.
For proper bind and operation it requires the MAX77686 PMIC driver.

New file: drivers/power/regulator/max77686.c
New config: CONFIG_DM_REGULATOR_MAX77686

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