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

Make mpddrc_base part of struct shdwc since there is also only one
instance of struct shdwc *at91_shdwc in system and to have all data
specific to SHDWC grouped together.

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>