power: reset: at91-poweroff: make sclk part of struct shdwc

Make sclk part of struct shdwc to have all the data specific to SHDWC
grouped together in one structure.

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>