remoteproc: fix a potential NULL-dereference on cleanup

We only need to allocate mapping if there is an IOMMU domain.

Otherwise, when the mappings are released, the assumption that
an IOMMU domain is there will crash and burn.

CC: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[ohad: revise commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>