iommu: Do not set COHERENT flag for secure pagetable allocation

Do not set COHERENT flag while allocating secure pagetables
using dma_alloc_attrs(), as this will eventually invoke
__dma_clear_buffer() which does dmac_flush_range() only
when COHERENT flag is false. This will prevent stale entries
which otherwise will lead to XPU violation in case of access
to this protected address range by hlos.

Change-Id: I3f9e646231c409f94801a334c0dd81adee524a0b
Signed-off-by: Archana Sriram <apsrir@codeaurora.org>