power: reset: at91-poweroff: introduce struct shdwc_reg_config

This driver uses AT91_PMC_MCKR in poweroff() function. But the
SAM9X60's PMC versions maps AT91_PMC_MCKR functionality at different
offset compared to the SAMA5D2's one. This patch prepares the field
so that different AT91_PMC_MCKR's offsets to be introduced in
struct reg_config so that proper offset to be used for AT91_PMC_MCKR
based on compatible string.

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>