ARM: at91: pm: keep at91_pm_backup_init() only for SAMA5D2 SoCs

In at91_pm_backup_init() return if it is not about SAMA5D2 SoCs.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>