iommu/amd: Add debugfs support to dump IOMMU MMIO registers

iommu/amd: Add debugfs support to dump IOMMU MMIO registers

Analyzing IOMMU MMIO registers gives a view of what IOMMU is
configured with on the system and is helpful to debug issues
with IOMMU.

eg.
-> To get mmio registers value at offset 0x18 for iommu<x> (say, iommu00)
   # echo "0x18" > /sys/kernel/debug/iommu/amd/iommu00/mmio
   # cat /sys/kernel/debug/iommu/amd/iommu00/mmio

Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Link: https://lore.kernel.org/r/20250702093804.849-3-dheerajkumar.srivastava@amd.com
Signed-off-by: Will Deacon <will@kernel.org>