iommu/debug: Add mutex to make attach/detach thread safe

This mutex lock will help to synchronise the usage of iommu domain
structure which is getting used by multiple threads to perform
iommu attach and iommu detach in case of any error.

Without this change we are seeing page poisoning - write after free
when running stress tests using iommu-debug test cases.

Change-Id: I1033808a510786577c41201c2a6a330e5465d4d1
Acked-by: Ankur Saxena <c_ankusa@qti.qualcomm.com>
Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>