rust: regulator: Add support for regmap

The regulator API offer many helpers to help simplifies drivers that
use the regmap API. This commit adds partial support for it, only the
function needed by the NCV6336 driver were added.

In case CONFIG_REGMAP=n, we create a dummy Regmap implementation in
order to avoid to have to surround a lot of the declarations and
statements with #[cfg(CONFIG_REGMAP)].

Signed-off-by: Fabien Parent <fabien.parent@linaro.org>

Author: Fabo