iommu/vt-d: Add the document for Intel IOMMU debugfs

This document guides users to dump the Intel IOMMU internals by debugfs.

Signed-off-by: Jingqi Liu <Jingqi.liu@intel.com>
Link: https://lore.kernel.org/r/20240207090742.23857-1-Jingqi.liu@intel.com


Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>