iommu/vt-d: Add debugfs support to show register contents

Add Intel IOMMU debugfs extension to dump all the register contents for
each IOMMU device.

Example:
root@OTC-KBLH-01:~# cat /sys/kernel/debug/iommu/intel/iommu_regset
IOMMU: dmar0 Register Base Address: fed90000
Name            Offset          Contents
VER             0x00            0x0000000000000010
CAP             0x08            0x01c0000c40660462
ECAP            0x10            0x0000000000f0101a
GCMD            0x18            0x0000000000000000
GSTS            0x1c            0x00000000c7000000
RTADDR          0x20            0x00000004071d3800
CCMD            0x28            0x0800000000000000
FSTS            0x34            0x0000000000000000
FECTL           0x38            0x0000000000000000
FEDATA          0x3c            0xfee0100400004021

Cc: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Gayatri Kammela <gayatri.kammela@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Co-Developed-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>