iommu: iommu-debug: Fix race conditions in iommu debug

With the current support, concurrent iommu debug calls
to the test device/debug domain can result in multiple
race conditions between attach, map, unmap and detach
calls.Hence introduce a new lock in the iommu_debug_device
structure to serialize iommu calls to the test device.

Change-Id: I94cd36eeaba4208972fdfa49ca4446a08d53b334
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>