ARM: at91: pm: change BU Power Switch to automatic mode

Change how the Backup Unit Power is configured and force the
automatic/hardware mode.
This change eliminates the need for software management of the power
switch, ensuring it transitions to the backup power source before
entering low power modes.

This is done in the only location where this switch was configured. It's
usually done in the bootloader.

Previously, the loss of the VDDANA (or VDDIN33) power source was not
automatically compensated by an alternative power source. This resulted
in the loss of Backup Unit content, including Backup Self-refresh low
power mode information, OTP emulation configuration, and boot
configuration, for instance.

Fixes: ac809e7879b1 ("ARM: at91: pm: switch backup area to vbat in backup mode")
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20241125165648.509162-1-nicolas.ferre@microchip.com
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>

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