rtc: pcf2127: add BSM support

The pcf2127 encodes BSM, BLD and power fail detection in the same set of
bits so it is necessary to do some calculation when changing BSM to keep
the rest of the configuration as-is. However, when BSM is disabled, there
is no configuration with BLD enabled so this will be lost when coming back
to a mode with BSM enabled.

Link: https://lore.kernel.org/r/20250127162728.86234-1-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>