Improvement on backup-switchover-mode overlay value definitions (#5884)

For the RV3028 RTC, the definitions for its `backup-switchover-mode` overlay
were not intelligible neither complete/exhaustive.

Accordingly to the https://github.com/raspberrypi/linux/issues/2912#issuecomment-477670051
these one here proposed should be correct.

`/boot/config.txt` should be as a configuration example, for rv3028, on a
 Uputronics GPS Extension HAT:

    # For GPS Expansion Board from Uputronics
    dtparam=i2c_arm=on
    dtoverlay=i2c-rtc,rv3028,backup-switchover-mode=3
    dtoverlay=pps-gpio,gpiopin=18
    init_uart_baud=115200

From my tests (`sudo rmmod rtc_rv3028 && sudo i2cget -y 1 0x52 0x37`):

`Default from factory`: `0x10`
`Mode 0`: `0x10`
`Mode 1`: `0x14`
`Mode 2`: `0x18`
`Mode 3`: `0x1c`

`Mode 3`: `0x1c` is consistent with the manufacturer configuration script: http://store.uputronics.com/files/configure-rv3028.sh