arm/arm64: dma-mapping: Fix iova region size

Clients may have additional mappings in an iommu domain which are not
managed by arm_iommu_create_mapping. As such, it is important that
arm_iommu_create_mapping only use the iova region specified by the client.
However, the current implementation rounds the size of the region up to a
256K boundary (on arm64).

Change-Id: I88ddd98a76b39e3e9126d78da8e725491f2a5b32
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>