ARM: at91: pm: wait for ddr power mode off

Wait for DDR power mode off before shutting down the core.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210415105010.569620-16-claudiu.beznea@microchip.com