regulator: Add ROHM BD9571MWV-M PMIC regulator driver

Add driver for the regulator block in the ROHM BD9571MWV-W MFD PMIC.
This block supports three voltage monitors, VD18, VD25, VD33 for the
1V8, 2V5, 3V3 voltage rails and a single voltage regulator for the
DVFS rail.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>