remoteproc: fix a potential NULL-dereference on cleanup

commit 7168d914a782086e217214c57ddfc7cc4b738c0c upstream.

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.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[ohad: revise commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>