power: reset: hisi-reboot: Unmap region obtained by of_iomap

Free memory mapping, if probe is not successful.

Fixes: 4a9b3737 ("power: reset: move hisilicon reboot code")
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>