iommu: arm-smmu: Take lock when doing unmap

We need to lock our calls to unmap since we have a performance
optimization which does child ref counting within the table entries and
if this ref count is updated in parallel, either through multiple unmaps
or multiple unmaps and maps, then its value can be corrupted leading to
page tables not getting freed or getting freed too early.

Change-Id: Ie38efd0b0c048f6de241601a6794f35c20b70787
Signed-off-by: Liam Mark <lmark@codeaurora.org>