power: reset: at91-poweroff: move shdwc related data to one structure

Move SHDWC realted data to only one structure to have them grouped.
Inspired from commit 9be74f0d ("power: reset: at91-poweroff: make
mpddrc_base part of struct shdwc").

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