rtc: rv8803: set VDETOFF and SWOFF via device tree

There might be designs where the power supply circuit is designed
in a way that VDETOFF and SWOFF is required to be set. Otherwise the
RTC detects a power loss. Add a device tree interface for this.

Signed-off-by: Carsten Resch <Carsten.Resch@de.bosch.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Oleksij Rempel <fixed-term.Oleksij.Rempel@de.bosch.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>