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

Use only one poweroff function and adapt it to work for both scenarios
(with LPDDR or not). The assignement of pm_power_off was moved at the
end of probe after all initializations are OK. This patch adapt the idea
from commit 4e018c1e ("power: reset: at91-poweroff: use only one
poweroff function").

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