Documentation: x86: rework IOMMU documentation

Add preliminary documentation for AMD IOMMU and combine
with the existing Intel IOMMU documentation and clean
up and modernize some of the existing documentation to
align with the current state of the kernel.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20220422192656.624920-1-alexander.deucher@amd.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>