rtc: pcf8523: add BSM support

Backup Switch Mode allows to select the strategy to use to switch from the
main power supply to the backup power supply. As before, the driver will
switch from standby mode to level mode but now only when it has never been
set.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211018153651.82069-5-alexandre.belloni@bootlin.com