riscv iommu pmap

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Summary

Add riscv page-management code (pmap) for IOMMU systems (similar to what we have on arm64).

Add CHERI Device-Model map/unmap routines as an example on how to use this pmap.

Test Plan

Tested with CHERI Device-Model
https://github.com/CTSRD-CHERI/device-model-riscv

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Fix macroses.

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Add iommu_pmap.h

[jrtc27]

I'm not sure this makes sense to upstream? And it'll conflict with whatever actual IOMMU spec ends up being added to RISC-V when they finally get round to it.

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In D32325#729404, @jrtc27 wrote:

I'm not sure this makes sense to upstream? And it'll conflict with whatever actual IOMMU spec ends up being added to RISC-V when they finally get round to it.

Agree but it will save a week or more for the person who will try to write this based on spec once it appear (Also a high probability that IO MMU will be compatible/similar to CPU MMU like in arm64).
We can just leave in disconnected to the build.
but I'm not insisting to have it in tree.

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Fix copyright year.