This commit adds the devicetree node "backup-switch-mode" to the rv3028
driver. It supports the following values:
- direct
- level
- disabled

The values correspond to the different BSM options in the datasheet.
When the setting is present in the devicetree, the driver checks the
current mode during the probing phase. If the desired value differs from
the current value, it is written to the RTC and a message is printed to
dmesg, stating the new mode.

Source: seco-ne/kernel/linux-imx-kuk@92daec58