backup-switchover-mode value definitions improvement

[tiagofreire-pt]

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

Accordingly to the raspberrypi/linux#2912 (comment), these should be correct.

[tiagofreire-pt]

/boot/config.txt should be:

Configurating 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```

[tiagofreire-pt]

From my tests (sudo i2cget -y 1 0x52 0x37):

Default from factory: 0x10

Mode 0: 0x10

Mode 1: 0x14

Mode 2: 0x18

Mode 3: 0x1c

[tiagofreire-pt]

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

[pelwell]

The README content looks reasonable, but it needs to patched at source - in our Linux repo: https://github.com/raspberrypi/linux

[tiagofreire-pt]

The README content looks reasonable, but it needs to patched at source - in our Linux repo: https://github.com/raspberrypi/linux

Done, as requested: raspberrypi/linux#5884

Thanks.