iommu: Remove unused of_get_dma_window()

iommu: Remove unused of_get_dma_window()

of_get_dma_window() was added in 2012 and removed in 2014 in commit
891846516317 ("memory: Add NVIDIA Tegra memory controller support").
Remove it and simplify the header to use forward declarations for
structs rather than includes.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Will Deacon <will@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: iommu@lists.linux-foundation.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210527193710.1281746-1-robh@kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>