ARM: at91: pm: switch backup area to vbat in backup mode

Backup area is now switched to VDDIN33 at boot (with the help of
bootloader). When switching to backup mode we need to switch backup area
to VBAT as all the other power sources are cut off. The resuming from
backup mode is done with the help of bootloader, so there is no need to
do something particular in Linux to restore backup area power source.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210830100927.22711-1-claudiu.beznea@microchip.com