iommu: debug: Fix leaky context banks

The contents of struct device->archdata contain private information
for the dma layer and should not be accessed by device drivers.

In this particular instance, the archdata->mapping field is set to
NULL after arm_iommu_detach_device(), leading to
arm_iommu_relase_mapping(NULL). This returns without freeing context
banks or other resources.

Change-Id: I974eb7ddc562e80c2d8a9e5260abf554e20042fa
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>