iommu/amd: Add documentation for AMD IOMMU debugfs support

Add documentation describing how to use AMD IOMMU debugfs support to
dump IOMMU data structures - IRT table, Device table, Registers (MMIO and
Capability) and command buffer.

Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Link: https://lore.kernel.org/r/20250702093804.849-9-dheerajkumar.srivastava@amd.com


Signed-off-by: Will Deacon <will@kernel.org>