rtc: rv3028: add BSM support

Backup Switch Mode controls how the RTC decides when to switch to the
backup power supply. As it is disabled by default, provide a way to enable
and configure it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211018151933.76865-7-alexandre.belloni@bootlin.com