rtc: rv3028: Mirror BSM and TCE/TCR to EEPROM

Periodically the RV3028 refreshes registers from the EEPROM. When this
happens, some settings that have only been committed to registers are
lost. Change the handling of backup-switchover-mode and
trickle-resistor-ohms to write the EEPROM as well (if something has
changed).

See: https://github.com/raspberrypi/linux/issues/2912



Signed-off-by: Phil Elwell <phil@raspberrypi.com>