Revert "iommu: arm-smmu: iommu_unmap_fast support"

This reverts commit 49d6dec0.
iommu_unmap_fast support require releasing cb_lock between unmap and tlb
flush. This allows room for a stale walk cache between unmap of last pte in
a pmd, which results in freeing of pmd. This followed by map on the same
pmd range results in valid access initiated on new mapping using stale walk
cache. The can be avoided by ensuring unmap and tlb flush happens while
keeping the cb_lock.

Change-Id: I2ad7e25ee858a2f70d6f9a9c53c66b54bd0df458
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>