power: reset: at91-poweroff: use proper master clock register offset

SAM9X60's PMC uses different offset for master clock register.
Add a member of type struct pmc_reg_config in struct reg_config,
fill it correspondingly for SAMA5D2 and SAM9X60 and use it in
poweroff() function.

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>