arm: make default iova alignment as 2mb

To get the 2MB aligned IOVA section mappings, set
ARM_DMA_IOMMU_ALIGNMENT to 9.

Change-Id: I3254c39e4864c32d6f25a4d416b1b7aec6756ecd
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>