power: reset: at91-poweroff: rename at91_shdwc_base member of struct shdwc

Rename at91_shdwc_base member of struct shdwc to shdwc_base. There is
already an "at91" string in at91_shdwc object.

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>