power: reset: at91-reset: add at91_reset_data

Add struct at91_reset_data to keep per platform related information.
This is a prerequisite for adding reset_controller_dev support.

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