mfd: Add ROHM BD9571MWV-M PMIC DT bindings

Add DT bindings for the ROHM BD9571MWV-M PMIC. This PMIC has
the following features:
- multiple voltage monitors for 1V8, 2V5, 3V3 voltage rail
- one voltage regulator for DVFS
- two GPIOs

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>