power: reset: at91-sama5d2_shdwc: fix wkupdbc mask

According to datasheet WKUPDBC mask is b/w bits 26..24.

Fixes: f80cb488 ("power: reset: at91-shdwc: add new shutdown controller driver")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>