regulator: Add driver for cros-ec-regulator

Add driver for cros-ec-regulator, representing a voltage regulator that
is connected and controlled by ChromeOS EC, and is controlled by kernel
with EC host commands.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200612040526.192878-4-pihsun@chromium.org


Signed-off-by: Mark Brown <broonie@kernel.org>