dt-bindings: regulator: Add DT binding for cros-ec-regulator

Add DT binding documentation for cros-ec-regulator, a voltage regulator
controlled by ChromeOS EC.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200612040526.192878-2-pihsun@chromium.org


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