arm64: make default ARM64_DMA_IOMMU_ALIGNMENT as 9

To realize section mapping of 2MB in arm64 iommu mappings,
set ARM64_DMA_IOMMU_ALIGNMENT to 9 so that 2MB aligned
virtual address can be allocated based on size.

2MB section mapping in iommu page tables would ensure less
pressure on TLBs.

Change-Id: Id22e935e113bee2a4ae7aeab15bcf17b7b6f3c78
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>