iommu/dma: Make header private

Now that dma-iommu.h only contains internal interfaces, make it
private to the IOMMU subsytem.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/b237e06c56a101f77af142a54b629b27aa179d22.1660668998.git.robin.murphy@arm.com


[ joro : re-add stub for iommu_dma_get_resv_regions ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>