lib: utils/reset: Register separate GPIO system reset devices

Now that sbi_system support multiple system reset devices, we should
register separate devices for GPIO restart and GPIO poweroff because
DT describes these as separate devices with different compatible
strings.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>