iommu/iova: Limit IOVA alignment using CONFIG_ARM64_DMA_IOMMU_ALIGNMENT

Currently IOVAs are aligned to the next power of two of the size of the
allocation. This can result in unnecessary fragmentation of the IOVA
space.

If defined use CONFIG_ARM64_DMA_IOMMU_ALIGNMENT to limit the maximum IOVA
alignment.

Change-Id: Ica7f6382237e37acac2d35972a564ca1e1f5e84f
Signed-off-by: Liam Mark <lmark@codeaurora.org>