power: reset: at91-reset: use devm_of_iomap

Use devm_of_iomap() to map resources. This will avoid the necessity to
track the mapped resources and free them on failure path or on remove.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>