iommu/arm-smmu: Protect the lists used for assign/unassign with a spinlock

Simultaneous assign or unassign on the same domain during map and unmap
operations manipulate the same pte_info lists and thus can ask hypervisor
to assign the same pte address twice which results in double mapping error.

The lists(pte_info_list and unassign_list) needs to be locked before
traversing it for making the assign/unassign calls.

Change-Id: Iebe3d703d80918af4ab18208f4b5a010c4c14791
Signed-off-by: Neeti Desai <neetid@codeaurora.org>