power: reset: at91-poweroff: use only one poweroff function

Use only one poweroff function for sama5d2 and adapt it to work for both
scenarios (having LPDDR or not).

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>