ARM: at91: pm: add backup mode support for SAMA7G5

Adapt at91_pm_backup_init() to work for SAMA7G5. Also, set the LPM pin
to shutdown controller. This will signal to PMIC that it needs to switch
to the state corresponding to backup mode.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210415105010.569620-21-claudiu.beznea@microchip.com